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z18n is a i18n library for Go. It can be used to translate Go applications and/or localize various aspects such as dates and numbers.

Import as API docs:

The chief motivation for writing this is that I wanted a nice painless API (as painless as i18n gets anyway), and none of the existing solutions that I could find really offered this, not without some extensive wrapping anyway.

It supports pluralisation, named variables with a convenient syntax, placeholder syntax for HTML tags, and localisation of dates and numbers.

README index:

Adding it to an application

Start by creating a new bundle; a "bundle" is a set of all translations your application has:

b, err := NewBundle(language.English)

This sets the default language set to English. The "default language" is the "native" language of the application. I will use English here, but there is nothing stopping you from writing an application in Russian and then translating that to English or other languages.

You add messages for a language:

b.AddMessages(language.English, map[string]Msg{
    "insult/cow":   Msg{Default: "You fight like a dairy farmer!"},
    "comeback/cow": Msg{Default: "How appropriate. You fight like a cow!"},

b.AddMessages(language.Dutch, map[string]Msg{
    "insult/cow":   Msg{Default: "Je vecht als een melkboer!"},
    "comeback/cow": Msg{Default: "Erg toepasselijk. Je vecht als een koe!"},

You can also load messages from TOML files; see the "Finding messages and creating translation files" section below.

To get messages you get a locale from the bundle:

l := b.Locale("nl-NL")

This accepts multiple languages, in order of preference, and accepts the contents of the Accept-Language HTTP header. In a real-world web app you usually want to do something like:

l := b.Locale(
    r.Query.Get("lang"),             // Prefer explicit overwrite from query param.
    user.Settings.Language,          // User setting in your application.
    r.Header.Get("Accept-Language")) // Last option is to use the browser setting.

Aside: please do not use the IP address for this. As someone lived abroad for a few years it's a massive PITA to have things automatically be set to a language I don't understand. Even in the Netherlands I often just prefer the English version.

If you have a CLI or desktop app you can use b.LocaleFromEnv() to create a new locale; this will use the LANG, LANGUAGES, and LC_* environment variables.

You almost certainly want to use codes with a region tag such as nl-NL when creating locales, nl being the language code for Dutch, and NL being the region of the Netherlands. It will fall back to the messages for nl if there aren't any for nl-NL specifically, and the localisation for dates and such will be appropriate for this region. American and British people don't write their dates in the same way for example, even though they both speak English.

You can use Locale.T to display translated messages:

// Output: "Erg toepasselijk. Je vecht als een koe!"

More details on how to translate messages in the section below.

A bundle only needs to be created once; if you're using this in a long-running webapp then create a bundle once on startup and a locale for every user/request based on the user settings, Accept-Language header, etc.

There is also a top-level z18n.T which takes the Locale object from a context, which can be created with z18n.With():

ctx := z18n.With(context.Background(), l)
z18n.T(ctx, "insult/cow")

Translating messages

The T function accepts the message ID as the first parameter:


This will look up the message with the ID song/coconuts in this locale. If no such message exists in this locale then it will try to get it from the closest locale such as nl.

This means you can translate messages in the nl language, which should be appropriate for most Dutch speakers, but also add a few regional variations for nl-BE (Dutch as spoken in Belgium) if need be.

A message ID can contain any printable character, including whitespace, but cannot start with a bar (|).

You can optionally specify a default message after a |:

l.T("song/coconuts|I've got a lovely bunch of coconuts!")

By adding both an ID and the message in there you get the best of both worlds: you can still easily find and write code without going back and forth to translation files, but you can also freely make minor changes to the default message without invalidating all the translations as the message itself isn't used as the lookup key.

You can only set the default (unpluralized) message with this; if you want a pluralized string then you will need to add the other variants as messages through an Bundle.AddMessages() call or message file (more on pluralisation and messages files later).

The / doesn't mean anything special, it's just a convention that might be useful. You can also use song-coconuts, song#coconuts, song coconuts, song/silly/coconuts, or just not use any prefix at all and use only coconuts. Personally I found using prefixes useful as it adds a bit context what something is (e.g. btn/accept clarifies this is a button).


Variable interpolation works with %(varname); the varname should remain identical in translated messages as it's used to look up the variable:

l.T("spam|Spam %(email) at your own risk; I will hunt you down!", email)

If you have only one variable then you can pass it as just an argument, but if if you have more than one you will need to use a map:

l.T("spam|Spam %(email) or %(email2) at your own risk; I will hunt you down!", z18n.P{
    "email":  email,
    "email2": email2,

The reason for this is that the position of the variables might be different in translated messages. With one variable this isn't an issue and since many messages will likely have just one variable it's a useful "shortcut" to have.

Variable names can contain any character except whitespace and any of %()[]|. After a space you can add one or more format specifiers:

lower, upper    Lower or uppercase everything.
upper_first     Uppercase the first letter, leaving the rest of the case intact.

html            Escape as HTML string; only has effect if Bundle.NoHTML is set.

raw             Don't format numbers or dates according to the locale.
                i.e. will print just "1000" instead of "1,000".

date, time      Print a time.Time as a date ("18-06-1985") or time ("17:15:30")
                Default is to print as datetime ("18-06-1985 17:15:30")

full, long      Print date, time, or datetime in  "full", "long", "medium",
medium, short   or "short" format. Defaults to "long"

day, month      Get the day or month name from a time.Time; combine with
                "short" to get an abbreviated name.

max             Set the minimum and maximum percision for floats.

[..]            Any other text for a time.Time is taken as a format string
                for time.Format.

For example:

%(word upper_first)         Format in upper case.
%(word lower upper_first)   Format in lower case, and then upper-case the first letter.

See the "Localisation of dates, numbers" section below for more details on number and date formatting.

Variable values are always HTML-escaped by default unless you set NoHTML in the Bundle. You almost certainly want to set this for CLI and desktop apps, it's enabled by default as forgetting to do so for webapps can be potentially disastrous, whereas it "only" looks wrong for a CLI or desktop app (but isn't dangerous).

HTML tags

Use %[varname text] as a placeholder for HTML tags; this is intended to be used with the z18n.Tag() function and removes the need for most – if not all – HTML inside translation strings, and makes stuff easier to read and HTML easier to update:

l.T("video/goat|Look at %[cute video] of a farting goat!")
    z18n.Tag("a", `href="/goat.mp4" class="link"`))

Everything inside the %[..] tag is text that should be translated. The z18n.Tag function controls which HTML gets added; the first parameter (a) is the tag name, and the second whatever you want to put in the opening tag; this is added without processing and is not safe against untrusted input.

As with %(..) variables you can pass it as just an argument if you have only one value, but will need to use a map and add a variable if you use multiple variables and/or tags:

l.T("video/goat|Look at %[%link cute video] of a farting %[%bold goat]!", z18n.P{
    "link": z18n.Tag("a", `href="/goat.mp4" class="link"`),
    "bold": z18n.Tag("strong", ``),

If the first word starts with % it's taken as the variable name; the % is added to distinguish between a variable name and a regular word.

You can use variables inside %[..] tags:

l.T("email/goat|You can %[%link email the goat at %(email)] for requests", z18n.P{
    "link":  z18n.Tag("a", `href=""`),
    "email": email,

It's not possible to nest these tags: %[%one %[%two tags]] won't work. You can create your own type which implements the z18n.Tagger interface if you need more complex HTML.

In some cases there isn't any text to be translated; in which case you can use the third parameter to set the innerHTML and use just a variable name:

l.T("email/goat|Or phone the goat at %(phone).",
    z18n.Tag("a", `href="tel:5554242"`, "555-42 42))

Localisation of dates, numbers

Numbers (all int and float types) and time.Time will be formatted according to the locale, for example:

l.T("test|The genetic test I did on %(t) showed I'm %(f)% platypus; a trait %(n) people share", z18n.P{
    "t": time.Now(),
    "f": 13.42,
    "n": 51341,

This will print the following for the en-US, en-NZ, and nl-NL locales:

The genetic test I did on Jun 12, 2021, 7:30:21 AM showed I'm 13.42% platypus; a trait 51,341 people share
The genetic test I did on 12/06/2021, 7:30:43 AM showed I'm 13.42% platypus; a trait 51,341 people share
The genetic test I did on 12 jun. 2021 07:32:57 showed I'm 13,42% platypus; a trait 51.341 people share

We can add some format specifiers to the variables to control how it's printed; for example adding 0 to the float to and short date to the time:

l.T("test|The genetic test I did on %(t short date) showed I'm %(f 0)% platypus; a trait %(n) people share", z18n.P{
    "t": time.Now(),
    "f": 13.42,
    "n": 51341,

Will print:

The genetic test I did on Saturday, June 12, 2021 showed I'm 13% platypus; a trait 51,341 people share
The genetic test I did on Saturday, 12 June 2021 showed I'm 13% platypus; a trait 51,341 people share
The genetic test I did on zaterdag 12 juni 2021 showed I'm 13% platypus; a trait 51.341 people share

Dates can be printed as "datetime" (the default), "date", or "time", all of them in a "full" "long", "medium", or "short" format. How exactly it's printed differs per language; a few examples of how it roughly looks:

                        en-US                       nl-NL

    %(d)                Jan 02, 2006 2:22 pm        2 Jan 2006 14:22
    %(d short)          02/01/06 2:22 pm            01-02-2006 14:22
    %(d long)           January 2, 2006 2:22 pm     2 January 2006 14:22

Or print just the date:
    %(d date)           02/01/06                    01-02-2006
    %(d date short)     January 2, 2006             2 January 2006
    %(d date long)      January 2, 2006             2 January 2006

Just the time
    %(d time)           2:22                        14:22

Extract specific parts:
    %(d day)            Monday                      Maandag
    %(d month)          March                       Maart

Or use a custom format:
    %(d 2006-01-02)     2006-01-02                  2006-01-02

Things like ordinals, formatting of bytes, etc. aren't implemented (yet).

Use the raw function in variables to prevent formatting and format it as fmt.Sprintf("%v"):

l.T("id|number: %(n raw); float: %(f raw); time: %(t raw)", z18n.P{
    "n": 1_230_495,
    "f": 6666.42,
    "t": time.Now(),


Thus far we've only set Msg.Default; this is the message to use if there are no pluralisations to apply; there are five other messages:

One, Zero, Two, Few, Many

z18n will use one of these (or the Default) automatically when supplied with a Plural value. Leaving the appropriate value empty in the message file will result in an error.

The exact meaning of these vary per language, and most languages don't have all of them. The logic for all of this can actually be quite complex and often includes exceptions – as languages do. Plurals in English (and most Germanic languages) are usually fairly easy with just "one" and "more than one", and you only need to set One and Default. Many Asian languages like Indonesian have it even easier by just not having plural forms at all, and Polish people must have a Ph.D. in math embedded in their DNA.

To add Plurals to the messages use the appropriate field(s):

b.AddMessages(language.BritishEnglish, map[string]Msg{
    "ants!": Msg{
        One:     "Help, I've got an ant in my trousers!"
        Default: "Help, I've got %(n) ants in my trousers!"

b.AddMessages(language.AmericanEnglish, map[string]Msg{
    "ants!": Msg{
        One:     "Help, I've got an ant in my pants!",
        Default: "Help, I've got %(n) ants in my pants!",

b.AddMessages(language.Indonesian, map[string]Msg{
    "ants!": Msg{
        // You can probably get away with just "Default" here; there is no
        // grammatical difference other than not putting the %(n) in there.
        One:     "Tolong, saya punya semut di celana saya! ",
        Default: "Tolong, saya punya %(n) semut di celana saya!",

b.AddMessages(language.Polish, map[string]Msg{
    "ants!": Msg{
        One:  "Pomocy, mam mrówkę w spodniach!",
        Two:  "Pomocy, mam %(n) mrówki w spodniach!",:
        Few:  "Pomocy, mam %(n) mrówek w spodniach!",:
        Many: "Pomocy, mam w spodniach %(n) mrówek!",:

Pass a z18n.Plural to z18n.T to tell z18n which form to use; z18n.N() conveniently creates this without too much noise:

l.T("ants!", z18n.N(5))

This can be in any position and will automatically be made available as the variable n, and can of course be combined with other variables:

b.AddMessages(language.BritishEnglish, map[string]Msg{
    "marketers": Msg{
        One:     "I emailed %(email) only once with my stupid marketing offer, so better send 5 more emails"
        Default: "I emailed %(email) %(n) times with my stupid marketing offers",

l.T("marketers", z18n.N(5), email)

Note that in CLDR "Default" is named "Other". I renamed this as I thought it made more sense, especially since most messages don't have any plurals/translations setting "Other" seems kinda weird.

Adding context

It's often useful for translators to have some clue what exactly a string refers to. Generally speaking, the shorter the string, the more useful adding context is.

There are two ways to add context; the first is in the message ID; for example:


This makes it clear that "get" is used as a form button to get a quote. This may be important, because words like "get" can sometimes be translated in multiple ways, and not all of them may be appropriate in this context. For example one product I worked one had "Get [product-name]" translated in such a way where "get" meant "to contract" (as in, "contract the flu"). Funny? Yes. A good translation? Not really.

This is one reason I like using prefixes, because now it's pretty clear that this is a button that does something (but you can also use get-quote-button or some other variation, if you prefer).

A second way is to use special comments:

// z18n: Context
// Context continues.


/* z18n: Context
   Context continues. */


l.T("button/get-quote|Get") // z18n: some context
l.T("button/get-quote|Get") /* z18n: some context */

I would recommend avoiding this unless necessary; good ids are better. But sometimes even with descriptive IDs it's useful to add some extra context.

The comments need to be prefixed with z18n: and immediately precede the T call. This won't work:

// z18n: there is a blank like.


// z18n There is no ":" after z18n.

//z18n: there is no space after //

// z18n: This is on the next line.

Using from templates

Using z18n from {text,html}/template is a first-class use case; you'll need to add a few functions:

tplfunc := template.FuncMap{
    "t":      z18n.Thtml,
    "tag":    z18n.Tag,
    "plural": z18n.N,

And then use:

{{t .Context "message/swedish|The Swedish prime minister has a massage for you."}}

Whitespace after at the start and end will be stripped and all other whitespace will be collapsed to a single space, so multi-line messages work well:

{{t .Context `message/swedish|
    The Swedish prime minister has a massage for you.

The downside of this is that you need to pass the context every time. You can create a "scoped" version by assigning a variable:

b := NewBundle(...)
l := b.Locale()

tpl.ExecuteTemplate("foo.gohtml", struct {
    T func(string, ...interface{}) template.HTML

And then use it like:

{{.T "message/swedish|The Swedish prime minister has a massage for you."}}

Or if there is only one user for the application (i.e. a CLI or desktop app) then you can use a regular FuncMap with a closure to achieve the same.

You can add context with {{/* z18n: ... */}} with the same rules as Go files.


z18n is a Go i18n tool, not a JavaScript one; there isn't great support for JavaScript right now.

That said, it's not uncommon to have an application where almost all of the text is in the backend with just a few messages in the frontend. The general strategy would be to render the messages you need server-side and then load them in JS. A simple example:

<span id="z18n" style="display: none"
    data-msg-one="{{.T "id/msg"}}
    data-msg-two="{{.T "id/msg"}}

And then get it from #z18n on init or when needed.

Or you can add a JSON endpoint and load it from there.

Support for some better/more convenient integration is something I plan to add later. There are actually quite a few i18n JS libraries out there already, and I need to see if any of them can be integrated.

You can, by the way, also use the Intl API to get people's locale preferences, which is more fine-grained than Accept-Language.

Finding messages and creating translation files

The ./cmd/z18n tool can find messages in Go and template files.

To find all messages in the current directory and all subdirectories:

% z18n init i18n

This will scan for Go files and templates. See z18n help for various options.

% z18n find > i18n/base.toml

To translate something, copy i18n/base.toml to i18n/file.toml, edit the information in the __meta__ key, and start translating!

To load messages from ReadMessages():

err := b.ReadMessages(os.DirFS("i18n"), "file.toml")

// Or all files in a directory:
err := b.ReadMessagesFromDir(os.DirFS("i18n"), "*.toml")

For updating use the update command:

% z18n update i18n/base.toml i18n/*.toml

The first file is the "base" file, and all other files are translation files. This will re-scan the project, add new translation entries, comment out entries that no longer exist, and mark entries where the default has changed.

The options used in "z18n find" are stored in the base file, so you don't need to add them again.


i18n library for Go







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