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i18n (v2)

Build Status

Smart internationalization for ASP.NET

    PM> Install-Package i18N


The i18n library is designed to replace the use of .NET resources in favor of an easier, globally recognized standard for localizing ASP.NET-based web applications.

Platforms Supported

i18n itself targets .NET Framework 4, and works with websites and web applications based on ASP.NET v4 and above, including:

  • ASP.NET WebMatrix / Web Pages
  • ASP.NET Web Forms


  • Leverages the GetText / PO ecosystem: localize like the big kids
  • Localize everything: HTML, Razor, C#, VB, JavaScript, .NET attributes and data annotations, ...
  • SEO-friendly: language selection varies the URL, and Content-Language is set appropriately
  • Automatic: no URL/routing changes required in the app
  • High performance, minimal overhead and minimal heap allocations
  • Unit testing support
  • Smart: knows when to hold them, fold them, walk away, or run, based on i18n best practices

Project Configuration

The i18n library works by modifying your HTTP traffic to perform string replacement and patching of URLs with language tags (URL Localization). The work is done by an HttpModule called i18n.LocalizingModule which should be enabled in your web.config file as follows:

      <add name="i18n.LocalizingModule" type="i18n.LocalizingModule, i18n" />
  <system.webServer> <!-- IIS7 'Integrated Mode'-specific config -->
      <add name="i18n.LocalizingModule" type="i18n.LocalizingModule, i18n" />

Note: The <system.web> element is added for completeness and may not be required.

The following <appSettings> are then required to specify the type and location of your application's source files:

    <add key="i18n.DirectoriesToScan" value=".." /> <!-- Rel to web.config file -->
    <add key="i18n.WhiteList" value="*.cs;*.cshtml;*.sitemap" />
    <add key="i18n.BlackList" value=".\js\kendo;.\js\angular;.\*\dist" />

The following configuration options are optional. i18n.DisableReferences allows you to generate lighter pot/po files by deleting references to your translation tokens (nuggets) and i18n.GenerateTemplatePerFile generates a pot file per file scanned and merges all po files into messages.po:

    <add key="i18n.DisableReferences" value="true" />
    <add key="i18n.GenerateTemplatePerFile" value="true" />

Certain behaviours of i18n may be altered at runtime on application startup. The following code shows the most common options:

    public class MvcApplication : System.Web.HttpApplication
        protected void Application_Start()
            // Change from the default of 'en'.
            i18n.LocalizedApplication.Current.DefaultLanguage = "fr";

            // Change from the default of 'i18n.langtag'.
            i18n.LocalizedApplication.Current.CookieName = "i18n_langtag";

            // Change from the of temporary redirects during URL localization
            i18n.LocalizedApplication.Current.PermanentRedirects = true;

            // This line can be used to disable URL Localization.
            //i18n.UrlLocalizer.UrlLocalizationScheme = i18n.UrlLocalizationScheme.Void;

            // Change the URL localization scheme from Scheme1.
            i18n.UrlLocalizer.UrlLocalizationScheme = i18n.UrlLocalizationScheme.Scheme2;

            // Change i18n's expectation for the ASP.NET application's virtual application root path on the server, 
            // used by Url Localization. Defaults to "/".
            //i18n.LocalizedApplication.Current.ApplicationPath = "/mysite";

            // Specifies whether the key for a message may be assumed to be the value for
            // the message in the default language. Defaults to true.
            //i18n.LocalizedApplication.Current.MessageKeyIsValueInDefaultLanguage = false;

            // Specifies a custom method called after a nugget has been translated
            // that allows the resulting message to be modified, for instance according to content type.
            // See [Issue #300]( for example usage case.
            i18n.LocalizedApplication.Current.TweakMessageTranslation = delegate(System.Web.HttpContextBase context, i18n.Helpers.Nugget nugget, i18n.LanguageTag langtag, string message)
                switch (context.Response.ContentType)
                    case "text/html":
                        return message.Replace("\'", "&apos;");
                return message;

            // Blacklist certain URLs from being 'localized' via a callback.
            i18n.UrlLocalizer.IncomingUrlFilters += delegate(Uri url) {
                if (url.LocalPath.EndsWith("sitemap.xml", StringComparison.OrdinalIgnoreCase)) {
                    return false; }
                return true;

            // Extend (+=) or override (=) the default handler for Set-PAL event.
            // The default handler applies the setting to both the CurrentCulture and CurrentUICulture
            // settings of the thread, as shown below.
            i18n.LocalizedApplication.Current.SetPrincipalAppLanguageForRequestHandlers = delegate(System.Web.HttpContextBase context, ILanguageTag langtag)
                // Do own stuff with the language tag.
                // The default handler does the following:
                if (langtag != null) {
                    Thread.CurrentThread.CurrentCulture = Thread.CurrentThread.CurrentUICulture = langtag.GetCultureInfo(); }

            // Blacklist certain URLs from being translated using a regex pattern. The default setting is:
            //i18n.LocalizedApplication.Current.UrlsToExcludeFromProcessing = new Regex(@"(?:\.(?:less|css)(?:\?|$))|(?i:i18nSkip|glimpse|trace|elmah)");

            // Whitelist content types to translate. The default setting is:
            //i18n.LocalizedApplication.Current.ContentTypesToLocalize = new Regex(@"^(?:(?:(?:text|application)/(?:plain|html|xml|javascript|x-javascript|json|x-json))(?:\s*;.*)?)$");

            // Change the types of async postback blocks that are localized
            //i18n.LocalizedApplication.Current.AsyncPostbackTypesToTranslate = "updatePanel,scriptStartupBlock,pageTitle";

            // Change which languages are parsed from the request, like skipping  the "Accept-Language"-header value. The default setting is:
            //i18n.HttpContextExtensions.GetRequestUserLanguagesImplementation = (context) => LanguageItem.ParseHttpLanguageHeader(context.Request.Headers["Accept-Language"]);

            // Override the i18n service injection. See source code for more details!
            //i18n.LocalizedApplication.Current.RootServices = new Myi18nRootServices();


To localize text in your application, surround your strings with [[[ and ]]] markup characters to mark them as translatable. That's it. Here's an example of localizing text in a Razor view:

    <div id="content">
        <h2>[[[Welcome to my web app!]]]</h2>
        <h3><span>[[[Amazing slogan here]]]</span></h3>
        <p>[[[Ad copy that would make Hiten Shah fall off his chair!]]]</p>
        <span class="button" title="[[[Click to see plans and pricing]]]">
            <a href="@Url.Action("Plans", "Home", new { area = "" })">
                <strong>[[[SEE PLANS & PRICING]]]</strong>
                <span>[[[Free unicorn with all plans!]]]</span>

And here's an example in an MVC controller:

    using i18n;
    namespace MyApplication
        public class HomeController : Controller
            public ActionResult Index()
                ViewBag.Message = "[[[Welcome to ASP.NET MVC!]]]";

                return View();

At last, you can localize your data annotations as easy as this:

    public class PasswordResetViewModel
        [Required(ErrorMessage="[[[Please fill in this field]]]")]
        [Email(ErrorMessage = "[[[Email not yet correct]]]")]
        [Display(Name = "[[[Email Address]]]")]
        public string Email

And localize arguments passed to MVC URL-Helpers or other functions that require a plain string:

@Html.LabelFor(m => m.Name, "[[[First Name]]]")

And for Javascript:

    <script type="text/javascript">
        $(function () {
            alert("[[[Hello world!]]]");


In PO terminology, strings you want to be translatable are known as messages. In i18n, messages are 'marked-up' in your source code as 'Nuggets'. The nugget markup allows i18n to filter the HTTP response looking for the message strings which are replaced with translated strings, where available. They also allow message strings to be located by the PostBuild PO file generator.

A simple nugget looks like this:

[[[translate me]]]

This defines a message with the key (aka msgid) of "translate me".

Nugget markup supports formated messages as follows:

string.Format("[[[welcome %1, today is %0|||{0}|||{1}]]]", day, name)

where the %0 and %1 tokens are replaced by the strings that replace the {0} and {1} items, respectively. (The reason for the extra level of redirection here is to facilitate the translator rearranging the order of the tokens for different languages.)

Nugget transformation supports translation of the arguments as follows:

[MaxLength(20, ErrorMessage="[[[%0 must be %1 characters or less|||(((CountryCode)))|||20]]]")]
public string CountryCode { get; set; }

where the Nugget markup will first replace (((CountryCode)) with the translated text and then merge the translated value into the main message.

Nugget markup supports comments (extracted comments in PO terminology) to be passed to the translator like so:

[[[translate me///this is an extracted comment]]]

And if you need to include the markup characters themselves within a message, you can HTML-escape them, for example:

[[[Please don't forget to add GoogleAd tags: [googleadsmall&#93;]]]

where &#93; is the HTML escape sequence for ]. The relevant escape sequences are:

  • / = &#47;
  • [ = &#91;
  • ] = &#93;
  • | = &#124;

See Issue #50 for more on Nuggets and why we have chosen to replace the GetText / _() style of marking-up messages.

Nugget markup customization

The character sequences for marking-up nuggets ([[[, ]]], |||, (((, ))) and ///) were chosen on the basis that they were unlikely to clash with common character sequences in HTML markup while at the same time being convenient for the programmer to enter (on most keyboards).

However, recognizing that a clash remains possible and nuggets thereby being falsely detected in source code or the HTML response, i18n allows you to define your own sequences for the markup which you know are not going to clash. You can configure these in web.config as follows:

    <add key="i18n.NuggetBeginToken" value="[&[" />
    <add key="i18n.NuggetEndToken" value="]&]" />
    <add key="i18n.NuggetDelimiterToken" value="||||" />
    <add key="i18n.NuggetCommentToken" value="////" />
    <add key="i18n.NuggetParameterBeginToken" value="(((" />
    <add key="i18n.NuggetParameterEndToken" value=")))" />

Message Visualization

i18n can be configured to visualize all processed messages. This is useful when testing your app to verify that all messages are tagged correctly. To enable this feature:

    <add key="i18n.VisualizeMessages" value="true" />
    <add key="i18n.NuggetVisualizeToken" value="!" />

When VisualizeMessages is active the NuggetVisualizeToken will be inserted at start and end of each translated message.

Two more optional parameters can be used to further customize the message visualization. i18n.VisualizeLanguageSeparator This enables display of the language tag that was use to localize each message. The language tag will be shown before each message, separated from the message by this parameter value. If the value is a blank string or the parameter is not present then language tags are not shown in message visualizations. i18n.NuggetVisualizeEndToken This allows for using different start and end tokens for visualizing messages. When this value is specified then the NuggetVisualizeToken will be inserted at start of each translated message and the NuggetVisualizeEndToken will be inserted at end of each translated message.

For example, to display language tags separated from messages by a colon, and add brackets to enclose the visualized messages, use the following message visualization configuration.

    <add key="i18n.VisualizeMessages" value="true" />
    <add key="i18n.VisualizeLanguageSeparator" value=":" />
    <add key="i18n.NuggetVisualizeToken" value="![" />
    <add key="i18n.NuggetVisualizeEndToken" value="]!" />

Message Context Support

i18n allows you to assign a msgctxt value to each message. The value of the msgctxt is taken from any comment you have defined in the nugget. This feature is optional and disabled by default. To enable this feature:

    <add key="i18n.MessageContextEnabledFromComment" value="true" />

Note that note all PO editors support msgctxt and indeed may be thrown by the value when present in .PO files. See Issue #90 for more details.

Multi-line messages

The PO spec supports messages that span multiple lines. i18n provides full support for these, simply by spreading the nugget over several lines.

For example, the following nugget is perfectly legal and should appear in your PO editor as a multi-line message:

[[[This is a
message spread over
three lines]]]

Static File Compression and i18n

The i18n module localizes nuggets in the HTTP response by modifying the response stream using a response filter (see the .NET Framework documentation for more info about the HttpResponse.Filter property). If the response stream is compressed before it reaches the i18n module then the module does not modify the stream. Currently the module is not designed to intercept static file requests before compression happens.

Two checks are implemented to ensure that the module does not modify compressed response streams:

  1. In i18n.LocalizingModule there is a check to see if the response Content-Encoding header is set to "gzip" and if it is then the module does not install the response filter.
  2. In i18n.ResponseFilter the stream content is checked for the presence of the gzip file format magic number (the first two bytes of a gzip file are set to 1F 8B). If the magic number is found at the beginning of the stream then the content is passed through without modification by the filter.

Because of the way that static file compression works in IIS, some responses to static files requests do not get compressed, so if you have static file compression enabled (it is enabled by default) AND you have nuggets within the content of a static file, then the response received by a client will be localized when the response is not compressed and it will not be localized when the response is compressed. In order to prevent this, it is important that you decide whether or not you will localize static files on your site because you need to do one of the following:

  1. If you want to use nuggets and localize static files - disable static file compression. This means that you will not get the benefit of the bandwidth savings of compressing static files, but if you are localizing static files then you have essentially taken the decision to make the static files dynamic.
  2. If you do not need to use nuggets and localize static files - leave static file compression enabled. You will now get the benefit of the bandwidth savings of compressing static files, but it is important that you must not put nuggets in the static files.

Note: Refer to Issue #163 for more on IIS compression settings.

Note: in some scenarios, it might be desirable to localize Javascipt (.js) files, making your application case 1. To easily disable static file compression, use the IIS manager and click on site name, Compression and uncheck "Enable static content compression".

Note: The Microsoft ScriptManager compresses responses to requests for ScriptResource.axd so these responses will always be compressed and the script that is returned by the ScriptManager will not be localized even if you disable static file compression.

Building PO databases

To set up automatic PO database building, add the following post-build task to your project, after adding i18n.PostBuild.exe as a project reference:

    "$(TargetDir)i18n.PostBuild.exe" "$(ProjectDir)\web.config"

You can find i18n.PostBuild.exe file under packages folder:


Alternatively, you may choose to install the i18n.POTGenerator.vsix Visual Studio extension (2012/2013). This installs an i18n button in the Solution Window for manual triggering of PO generation. Note that it is necessary to highlight the project in question within the Solution Window before pressing the button.

The PO generator will rip through your source code (as defined by the i18n.DirectoriesToScan and i18n.WhiteList settings in web.config), finding every nugget, and uses this to build a master .POT template file located at locale/messages.pot relative to your web application folder. After the new template is constructed, any locales that exist inside the locale folder (or as defined by the i18n.AvailableLanguages semi-colon-delimited web.config setting) are automatically merged with the template, so that new strings can be flagged for further translation.

From here, you can use any of the widely available PO editing tools (like POEdit) to provide locale-specific text and place them in your locale folder relative to the provided language, e.g. locale/fr. If you change a PO file on the fly, i18n will update accordingly; you do not need to restart your application. Note that the locale-specific file must be named messages.po. For example, your locale folder structure will be similar to (three languages, fr, es, and es-MX are defined):


Custom Modifications To Translations

Nuggets translations can be modified at runtime as follows:

    protected void Application_Start()
        // Specifies a custom method called after a nugget has been translated
        // that allows the resulting message to be modified, for instance according to content type.
        // See [Issue #300]( for example usage case.
        i18n.LocalizedApplication.Current.TweakMessageTranslation = delegate(System.Web.HttpContextBase context, i18n.Helpers.Nugget nugget, i18n.LanguageTag langtag, string message)
            switch (context.Response.ContentType)
                case "text/html":
                    return message.Replace("\'", "&apos;");
            return message;

PO customization

i18n allows you to change the PO file name to use and use PO files from other sources (when working with multiple projects for example). To enable this feature, you can set :

    <add key="i18n.LocaleFilename" value="messages" />
    <add key="i18n.LocaleOtherFiles" value="external1;external2" /><!-- relative path from the directory of {LocaleFilename}.po-->

Note : i18n.LocaleOtherFiles paths are relative to the directory of the file {i18n.LocaleFilename}.po (messages.po by default).

URL Localization

In keeping with emerging standards for internationalized web applications, i18n provides support for localized URLs. For example, or

Out of the box, i18n will attempt to ensure the current language for any request is shown correctly in the address box of the user's browser, redirecting from any non-localized URL if necessary to a localized one. This is known as Early URL Localization. See also Principal Application Language.

While URLs from the user-agent perspective are localized, from the app's perspective they are nonlocalized. Thus you can write your app without worrying about the language tag in the URL.

The default URL Localization scheme (Scheme1) will show the language tag in the URL always; an alternative scheme, Scheme2, will show the language tag only if it is not the default.

Disabling URL Localization

URL localization can be disabled by setting the scheme to i18n.UrlLocalizationScheme.Void in Application_Start:

    protected void Application_Start()
        // Disable URL Localization.
        i18n.UrlLocalizer.UrlLocalizationScheme = i18n.UrlLocalizationScheme.Void;

Without URL localization, i18n will rely on the cookie "i18n.langtag" to determine the current language for each request. This means that the language change/setting feature on your site should change the cookie and set the new PrincipalAppLanguage:

  HttpCookie c = new HttpCookie("i18n.langtag") { 
    Value = Request.QueryString("newLanguage"), 
    HttpOnly = true, 
    Expires = DateTime.UtcNow.AddYears(1) 
  i18n.ILanguageTag p = default(i18n.ILanguageTag);
  p = i18n.LanguageTag.GetCachedInstance(Request.QueryString("newLanguage"));
  i18n.HttpContextExtensions.SetPrincipalAppLanguageForRequest(this.Context, p);

If you are experiencing problems with static content, maybe also related to browser caching and are having trouble getting the rules for URL exclusion in the following paragraphs to work, the Void scheme might we worth looking into. Please see Issue #385.

Exclude URLs from being localized

URLs to non-internationalized resources need not be localized. Typically, there is no harm in them being localized as i18n will route the request approriately either way. However, where the Principal Application Language for a request is not required, such as for when reading a CSS file or font file, it can save a redirection round trip by instructing i18n NOT to localize the URL.

There are two ways to instruct i18n NOT to localize a URL:

Firstly, you can set a RegEx pattern to match against the localpath part of the URLs to be excluded. For instance:

    protected void Application_Start()
        // Blacklist certain URLs from being 'localized'.
        i18n.UrlLocalizer.QuickUrlExclusionFilter = new System.Text.RegularExpressions.Regex(@"(^\/api\/)|((sitemap\.xml|\.css|\.less|\.jpg|\.jpeg|\.png|\.gif|\.ico|\.svg|\.woff|\.woff2|\.ttf|\.eot)$)", RegexOptions.IgnoreCase);

Indeed, the default value for the QuickUrlExclusionFilter settings is as shown above however feel free to override or set to null to disable.

For finer control, the second method is to define filter delegates that are passed the URL and return true if the URL is to be localized, otherwise false. For example:

    protected void Application_Start()
        // Blacklist certain URLs from being 'localized'.
        i18n.UrlLocalizer.IncomingUrlFilters += delegate(Uri url) {
            if (url.LocalPath.EndsWith("sitemap.xml", StringComparison.OrdinalIgnoreCase)) {
                return false; }
            return true;
        i18n.UrlLocalizer.OutgoingUrlFilters += delegate(string url, Uri currentRequestUrl) {
            Uri uri;
            if (Uri.TryCreate(url, UriKind.Absolute, out uri)
                || Uri.TryCreate(currentRequestUrl, url, out uri)) {
                if (uri.LocalPath.EndsWith("sitemap.xml", StringComparison.OrdinalIgnoreCase)) {
                    return false; }
            return true;

Conditionally ignore localization for a specific URL

There are very rare cases where you need to conditionally bypass the URL localization for a specific URL. One example is when generating hreflang tags when using i18n with Scheme2.

You can do this by prefixing the URL like so:

    <link rel="alternate" hreflang="en" href="@(EarlyUrlLocalizer.IgnoreLocalizationUrlPrefix)" />
    <link rel="alternate" hreflang="fr" href="" />
    <link rel="alternate" hreflang="es" href="" />

When i18n goes through the process for localizing outgoing URLs, this prefix will be stripped and the rendered URL will be left non-localized.

Note that this method of ignoring URL localization should not be widespread and is included to address edge cases. Most use cases that require ignoring URL localization can be solved more eloquently by making use of the UrlLocalizer filters.

Principal Application Language

During startup of your ASP.NET application, i18n determines the set of application languages for which one or more translated messages exist.

Then, on each request, one of these languages is selected as the Principal Application Language (PAL) for the request.

The PAL for the request is determined by the first of the following conditions that is met:

For i18n.UrlLocalizationScheme.Scheme1:

  1. The path component of the URL is prefixed with a language tag that matches exactly one of the application languages. E.g. "".

  2. The path component of the URL is prefixed with a language tag that matches loosely one of the application languages (see below).

  3. The request contains a cookie called "i18n.langtag" with a language tag that matches (exactly or loosely) one of the application languages.

  4. The request contains an Accept-Language header with a language that matches (exactly or loosely) one of the application languages.

  5. The default application language is selected (see also Per-Request Default Language Determination).

For i18n.UrlLocalizationScheme.Scheme2:

  1. The path component of the URL is prefixed with a language tag that matches exactly one of the application languages. E.g. "".

  2. The path component of the URL is prefixed with a language tag that matches loosely one of the application languages (see below).

  3. The default application language is selected (see also Per-Request Default Language Determination).

Where a loose match is made above, the URL is updated with the matched application language tag and a redirect is issued. E.g. "" -> "". By default this is a temporary 302 redirect, but you can choose for it to be a permanent 301 one by setting i18n.LocalizedApplication.Current.PermanentRedirects = true in Application_Start.

The GetPrincipalAppLanguageForRequest extension method to HttpContext can be called to access the PAL of the current request. For example, it may be called in a Razor view as follows to display the current langue to the user:

    @using i18n

        <p id="lang_cur" title="@Context.GetPrincipalAppLanguageForRequest()">

Similarly, the HTML lang attribute can be set as follows:

    @using i18n

    <html lang="@Context.GetPrincipalAppLanguageForRequest()">

Per-Request Default Language Determination

When the PAL algorithm falls back on the default language for the application, i18n supports a simple delegate-based hook for providing the default language based on the current request, typically based on the URL.

For example, suppose you wish the default language to vary as follows:

  1. -> 'en'
  2. -> 'fr'

This can be achieved as follows:

    protected void Application_Start()
        i18n.LocalizedApplication.Current.DefaultLanguage = "en";
        i18n.UrlLocalizer.UrlLocalizationScheme = i18n.UrlLocalizationScheme.Scheme2;
        i18n.UrlLocalizer.DetermineDefaultLanguageFromRequest = delegate(HttpContextBase context)
            if (context != null && context.Request.Url.Host.EndsWith(".fr", StringComparison.OrdinalIgnoreCase)) {
                return i18n.LanguageTag.GetCachedInstance("fr"); }
            return i18n.LocalizedApplication.Current.DefaultLanguageTag;

Notice how the URL localization scheme has been switched to Scheme2 which allows the URL to be without any language tag. The default scheme (Scheme1) would enforce a redirection so that the URL always contains the current language tag.

Explicit User Language Selection

You can provide a language selection feature in your application using i18n. There are two parts to implementing this feature which revolve around the setting of a cookie called i18n.langtag.

Firstly, provide HTML that displays the current language and allows the user to explicitly select a language (from those application languages available).

An example of how to do that in ASP.NET MVC and Razor follows:

@using i18n
<div id="language">
    <p id="lang_cur" title="@Context.GetPrincipalAppLanguageForRequest()">@Context.GetPrincipalAppLanguageForRequest().GetNativeNameTitleCase()</p>
  <div id="lang_menu" style="display: none;">
    <table class="table_grid">
          int i;
          int maxcols = 3;
          KeyValuePair<string, i18n.LanguageTag>[] langs = LanguageHelpers.GetAppLanguages().OrderBy(x => x.Key).ToArray();
          int cellcnt = langs.Length +1;
          for (i = 0; i < cellcnt;) {
            bool lastRow = i + maxcols >= cellcnt;
            <tr class="@(Html.Raw((i % 2) == 0 ? "even":"odd")) @(Html.Raw(lastRow ? "last":""))">
              @for (int j = 0; j < maxcols && i < cellcnt; ++i, ++j) {
                string langtag;
                string title;
                string nativelangname;
                if (i == 0) {
                  langtag = "";
                  title = "[[[Browser default language setting]]]";
                  nativelangname = "[[[Auto]]]";
                else {
                  i18n.LanguageTag lt = langs[i -1].Value;
                  title = langtag = lt.ToString();
                  nativelangname = lt.NativeNameTitleCase;
                    linkText: nativelangname, 
                    actionName: "SetLanguage", 
                    controllerName: "Account", 
                    routeValues: new { langtag = langtag, returnUrl = Request.Url },
                    htmlAttributes: new { title = title } )
              @* Fill last row with empty cells if ness, so that borders are added and balanced out. *@
              @if (lastRow) {
                for (; i % maxcols != 0; ++i) {

On selection of a language in the above code, the AccountController.SetLanguage method is called. For example:

    using i18n;

    // GET: /Account/SetLanguage

    public ActionResult SetLanguage(string langtag, string returnUrl)
        // If valid 'langtag' passed.
        i18n.LanguageTag lt = i18n.LanguageTag.GetCachedInstance(langtag);
        if (lt.IsValid()) {
            // Set persistent cookie in the client to remember the language choice.
            Response.Cookies.Add(new HttpCookie("i18n.langtag")
                Value = lt.ToString(),
                HttpOnly = true,
                Expires = DateTime.UtcNow.AddYears(1)
        // Owise...delete any 'language' cookie in the client.
        else {
            var cookie = Response.Cookies["i18n.langtag"];
            if (cookie != null) {
                cookie.Value = null;
                cookie.Expires = DateTime.UtcNow.AddMonths(-1);
        // Update PAL setting so that new language is reflected in any URL patched in the 
        // response (Late URL Localization).
        // Patch in the new langtag into any return URL.
        if (returnUrl.IsSet())
            if (LocalizedApplication.Current.UrlLocalizerForApp.FilterOutgoing(returnUrl, HttpContext.Request.Url)) // if url wants to be localized
                returnUrl = LocalizedApplication.Current.UrlLocalizerForApp.SetLangTagInUrlPath(HttpContext, returnUrl, UriKind.RelativeOrAbsolute, lt?.ToString()).ToString();
        // Redirect user agent as approp.
        return this.Redirect(returnUrl);

How to get a translation of a nugget in your C# code

With i18n you can access the translation for a given nugget msgid from any code that is handling an ASP.NET request. There is a GetText extension method to HttpContextBase provided for this.

For example, you can do the following from within an MVC controller action:

using System;
using System.Web.Mvc;
using i18n;

namespace MyWebSite.Controllers
    public class MyController : Controller
        public ActionResult Welcome()
            string welcomeMessage = HttpContext.GetText("Welcome to the my website.", "");

            // Do something with the string...

            return View();

Essentially, anywhere you have access to an HttpContextBase or HttpContext instance, you can get a correct translation for a given nugget msgid / msgcomment combo.

The msgcomment is relevant only when i18n.Domain.Concrete.i18nSettings.MessageContextEnabledFromComment is set to true; by default it is false and so msgcomment argument should be passed as null or empty.

Furthermore, you can access the translation of a complete body of text containing zero or more nuggets that require parsing using the ParseAndTranslate extension method to HttpContextBase, as follows:

    string entity = HttpContext.ParseAndTranslate("Hi - [[[Sign in]]]");

or if outside of an HttpContext, for example a background job running an emailing service task:

    string entity = i18n.LanguageHelpers.ParseAndTranslate("[[[Thank you for your payment]]]");

which will translate using the app's default language (i18n.LocalizedApplication.DefaultLanguage), or

    string entity = i18n.LanguageHelpers.ParseAndTranslate("[[[Thank you for your payment]]]", "fr-CA;q=1,fr;q=0.5");

which will use the app language that best matches those specified, or better still

    // During earlier HTTP request from user, save their language(s).
    string userLanguages = HttpContext.GetRequestUserLanguagesAsString();

    // Switch to background job.
    HostingEnvironment.QueueBackgroundWorkItem(ct => {

        // Translate using user's languages obtained earlier.
        string entity = i18n.LanguageHelpers.ParseAndTranslate("[[[Thank you for your payment]]]", userLanguages);


which will match against the languages obtained from the user's browser at some point earlier.

Language Matching

Language matching is performed when a list of one or more user-preferred languages is matched against a list of one or more application languages, the goal being to choose the application languages which the user is most likely to understand. The algorithm for this is multi-facted and multi-pass and takes the Language, Script and Region subtags into account.

Matching is performed once per-request to determine the Principal Application Language for the request, and also once per message to be translated (aka GetText call). The multi-pass approach ensures a thorough attempt is made at matching a user's list of preferred languages (from their Accept-Language HTTP header). E.g. in the context of the following request:

User Languages: fr-CH, fr-CA  
Application Languages: fr-CA, fr, en

fr-CA will be matched first, and if no resource exists for that language, fr is tried, and failing that, the default language en is fallen back on.

In recognition of the potential bottleneck of the GetText call (which typically is called many times per-request), the matching algorithm is efficient for managed code (lock-free and essentially heap-allocation free).

Note that the following Chinese languages tags are normalized: zh-CN to zh-Hans, and zh-TW to zh-Hant. It is still safe to use zh-CN and zh-TW, but internally they will be treated as equivalent to their new forms.

Private Use Subtag

The w3c language tag spec includes a provision for an additional subtag for private use. This is now supported and can be used to provide a different translation for specific scenarios, such as a tenant on a multi-tenant application.

The format is: en-GB-x-Tenant123, en-x-Tenant99 etc.

Note the -x-, after which you can add four or more alphanumeric characters to specify your custom translation. There must be an exact match for all subtags for this translation to be returned. If the module can't find a translation for the tenant, it will match the remaining subtags according to the algorithm described above.

Microsoft Pseudo-Locales and App Testing

As an aid to testing the localization of you app, Microsoft have added some 'pseudo-locales' to Windows.

Specifically, these are identified by the following special language tags qps-ploc, qps-plocm and qps-ploa.

i18n supports the use of these special locales. See Issue #195 for further details.

Language Matching Update

The latest refinement to the language matching algoritm:

// Principle Application Language (PAL) Prioritization:
//   User has selected an explicit language in the webapp e.g. fr-CH (i.e. PAL is set to fr-CH).
//   Their browser is set to languages en-US, zh-Hans.
//   Therefore, UserLanguages[] equals fr-CH, en-US, zh-Hans.
//   We don't have a particular message in fr-CH, but have it in fr and fr-CA.
//   We also have message in en-US and zh-Hans.
//   We presume the message from fr or fr-CA is better match than en-US or zh-Hans.
//   However, without PAL prioritization, en-US is returned and failing that, zh-Hans.
//   Therefore, for the 1st entry in UserLanguages (i.e. explicit user selection in app)
//   we try all match grades first. Only if there is no match whatsoever for the PAL
//   do we move no to the other (browser) languages, where return to prioritizing match grade
//   i.e. loop through all the languages first at the strictest match grade before loosening 
//   to the next match grade, and so on.
// Refinement to PAL Prioritization:
//   UserLanguages (UL) = de-ch,de-at (PAL = de-ch)
//   AppLanguages  (AL) = de,de-at,en
//   There is no exact match for PAL in AppLanguages.
//   However:
//    1. the second UL (de-at) has an exact match with an AL
//    2. the parent of the PAL (de) has an exact match with an AL.
//   Normally, PAL Prioritization means that 2. takes preference.
//   However, that means choosing de over de-at, when the user
//   has said they understand de-at (it being preferable to be
//   more specific, esp. in the case of different scripts under 
//   the same language).
//   Therefore, as a refinement to PAL Prioritization, before selecting
//   'de' we run the full algorithm again (without PAL Prioritization) 
//   but only considering langtags related to the PAL.

UpdatePanel / Async Postbacks / Partial Page Rendering

Responses to UpdatePanel async postback requests are handled as a special case because the content of the response is a set of formatted blocks, which may or may not contain partial segments of text or HTML that need to be localized. Each formatted block has the following structure


By default, only blocks with a type of updatePanel, scriptStartupBlock, or pageTitle get localized. You can localize segments in other block types by changing the value of AsyncPostbackTypesToTranslate in Application_Start. For example, to include the hiddenField blocks, add the following to Application_Start

i18n.LocalizedApplication.Current.AsyncPostbackTypesToTranslate = "updatePanel,scriptStartupBlock,pageTitle,hiddenField";

OWIN support

Support for OWIN is available to a limited extent. See issues #241 and #333 for more details. i18n is created based on HttpContextBase in System.Web assembly, which means the foundation was built on IIS pipeline. Currently we support OWIN hosted in IIS only, so it is still dependent on System.Web. Self-hosted OWIN is not supported.

Here is how to use i18n in OWIN Web API projects:

  • Add reference to i18n.Adapter.OwinSystemWeb (available on NuGet as well)
  • Add reference to Microsoft.Owin.Host.SystemWeb. If you add i18n.Adapter.OwinSystemWeb from NuGet it should automatically add this for you.
  • No need to register HttpModule in web.config file.
  • Add the following middleware registration into your startup sequence:
public partial class Startup
    public void Configuration(IAppBuilder app)

        // i18n config
        i18n.LocalizedApplication.Current.DefaultLanguage = "en";

        // i18n middleware

        // i18n response filter installer for static files
        var staticFileOptions = new StaticFileOptions
            OnPrepareResponse = (staticFileResponseContext) =>
                if (staticFileResponseContext.File.Name.EndsWith(".js", StringComparison.OrdinalIgnoreCase))
                    HttpContextBase context = staticFileResponseContext.OwinContext.Get<HttpContextBase>(typeof(HttpContextBase).FullName);

  • Add the following handler to Global.asax:
    /// <summary>
    /// Handles the ReleaseRequestState event of the Application control.
    /// </summary>
    /// <param name="sender">The source of the event.</param>
    /// <param name="e">The <see cref="EventArgs"/> instance containing the event data.</param>
    protected void Application_ReleaseRequestState(object sender, EventArgs e)
        HttpContextBase context = this.Request.GetOwinContext().Get<HttpContextBase>(typeof(HttpContextBase).FullName);

A reminder about folders in a web application

Your locale folder is exposed to HTTP requests as-is, just like a typical log directory, so remember to block all requests to this folder by adding a Web.config file.

    <?xml version="1.0"?>
                <add path="*" verb="*" type="System.Web.HttpNotFoundHandler"/>
                <remove name="BlockViewHandler"/>
                <add name="BlockViewHandler" path="*" verb="*" preCondition="integratedMode" type="System.Web.HttpNotFoundHandler"/>

Unit Testing With i18n

i18n provides the i18n.ITranslateSvc interface that abstracts the basic operation of parsing and translating a string entity that may contain one or more nuggets:

    public interface ITranslateSvc
        string ParseAndTranslate(string entity);

The following stock implementations of i18n.ITranslateSvc are provided by the library:

  • TranslateSvc_Invariant - ITranslateSvc implementation that simply passes through the entity (useful for testing).
  • TranslateSvc_HttpContextBase - ITranslateSvc implementation based on an given HttpContextBase instance.
  • TranslateSvc_HttpContext - ITranslateSvc implementation based on an given HttpContext instance.
  • TranslateSvc_HttpContextCurrent - ITranslateSvc implementation based on the static HttpContext.Current instance (obtained at the time of calling the interface).


There's lot of room for further enhancements and features to this library, and you are encouraged to fork it and contribute back anything new. Specifically, these would be great places to add more functionality:

  • Full OWIN support (see Issue #241)
  • Input and ideas on a safe universal nugget syntax (see issue #69).
  • Plurals support.
  • Help me fix the bugs! Chances are I don't ship in your language. Fix what hurts. Please?

Coding Style Guidlines

  • Pull Requests that add functionality to be accompanied with documentation added to this README.
  • Pull Request to be as granular as possible (e.g. limited to single features/enhancements).
  • All methods to be commented including helper routine.
  • 4-spaces used for tab indent.

Line Endings

The i18n project has adopted the GitHub recommendation with regard to standardized line endings in text files. Specifically, text files are stored in the Git index with line endings respresented by the single LF character (not CR/LF).

That means that, for Windows clients, you will probably want Git to convert line endings to CR/LF when checking text files out of the index, and converting them back to LF line endings when committing in. This behaviour is controlled via Git's core.autocrlf setting, which in this case would be set to true.

See Dealing with line endings for more information.

Build Notes

The i18n project at present targets .NET Framework 4 and later. To build i18n from source, Visual Studio 2019 or later is recommended.

Known Issues

  • MVC controller names must be more than 3 chars (#370).

Release History

2.1.17 (20201209)

  • FIX: Only every other nugget in messages.po is being translated (#413). Fix for a regression introduced in v2.1.16.

2.1.16 (20201126)

  • ADDED: Enhanced support for translations invoked by background jobs (LanguageHelpers.ParseAndTranslate + HttpContextExtensions.GetRequestUserLanguagesAsString + IBackgroundTranslateSvc).
  • ADDED: i18n project automated build and test (Continuous Integration) with github actions.
  • FIX: i18n.LanguageTag.ExtractLangTagFromUrl returns/outputs incorrect values when passed an absolute url.
  • FIX: Problem with HttpContext.ParseAndTranslate helper method (#338).
  • FIX: ArgumentNullException if the Content-Type header is not set in the response (#337).
  • FIX: Missing PO comment lines breaks translation (#351).

2.1.15 (20190814)

  • FIX: NullReferenceException caused by bad langtag (#387).
  • FIX: LangTag extraction logic broken by URL with query string immediately after lantag (#383).

2.1.14 (20180710)

  • FIX: Localization of outgoing URIs" feature issue in version 2.1.13 (#374).

2.1.13 (20180707)

  • FIX: performance issues related to translations in the default application language (#368).
  • FIX: URI fragments breaking localization of outgoing URIs (#372).

2.1.11 (20180528)

  • Improved support for wildcards in BlackList and WhileList settings (#319).
  • FIX: redundant updates to PO files (#329).
  • Modifications to OWIN support (#334) [BREAKING CHANGE].

2.1.10 (20161206)

  • New setting i18n.DisableReferences allows for the generation of lighter POT/PO files by excluding nugget references (#304).
  • New setting i18n.GenerateTemplatePerFile enables the breakdown of the POT template file into one POT file per scanned file (#314).
  • FIX: PostBuild bug introduced by release 2.1.9 (#316).
  • FIX: Duplicate message properties in POT/PO files.
  • Introduced support for publishing regular pre-release packages to NuGet.

2.1.9 (20161125)

  • Support for customization of PO filenames and sources (#305).
  • Support for modifying nugget translation at runtime (#300).
  • Support for changing default i18n cookie name (#296).
  • Added /api/ to default UrlLocalizer.QuickUrlExclusionFilter (#289).
  • Support for converting outgoing URLs where un-rooted paths into rooted paths (common in ASP.NET WebForms) (#286).

2.1.8 (20160807)

  • Support for ignoring Accept-Language request header (#278, #285).
  • Support for optionally showing source context next to reference paths & line numbers (#268)


Among the many contributors to the i18n library, a special acknowledgement is due to Daniel Crenna who originated this project.