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webL10n is a client-side, cross-browser i18n/l10n library (internationalization / localization), designed with modern web applications in mind.

Unlike other i18n/l10n libraries, webL10n supports:

  • declarative localization: elements with l10n-* attributes are automatically translated when the document is loaded;
  • named variables instead of printf-like %s tokens;
  • a simple and full-featured pluralization system;
  • server-less language negotiation (think of offline webapps);

We don’t focus on the gettext format: the bullet-proof *.properties format, used in Mozilla and GWT projects, is preferred — at least, by default.

Demo: (outdated — feel free to submit a new one! ^^)

This library is also used by the FirefoxOS front-end (Gaia) with a different JavaScript API: a modern API for FirefoxOS/Gaia (with web standards in mind), a basic cross-browser one in webL10n (compatibility with IE6/IE7/IE8).

More information on the Wiki.

Quick Start

Here’s a quick way to get a multilingual HTML page:

  <script type="text/javascript" src="l10n.js"></script>
  <link rel="prefetch" type="application/l10n" href="data.ini" />
  <button data-l10n-id="test" title="click me!">This is a test</button>
  • l10n resource files are associated to the HTML document with a <link> element;
  • translatable elements carry a data-l10n-id attribute;
  • l10n resources are stored in a bullet-proof *.ini file:
test = This is a test
test.title = click me!
test = Ceci est un test
test.title = cliquez-moi !

JavaScript API

l10n.js exposes a rather simple document.webL10n object.

// Set the 'lang' and 'dir' attributes to <html> when the page is translated
window.addEventListener('localized', function() {
  document.documentElement.lang = document.webL10n.getLanguage();
  document.documentElement.dir = document.webL10n.getDirection();
}, false);
  • localized event: fired when the page has been translated;
  • getLanguage / setLanguage method: get/set the ISO-639-1 code of the current locale;
  • getDirection method: direction (ltr|rtl) of the current language;
  • get method: get a translated string.
var message = document.webL10n.get('test');

You will probably use a gettext-like alias:

var _ = document.webL10n.get;

To handle complex strings, the get() method can accept optional arguments:

alert(_('welcome', { user: "John" }));

where welcome is defined like this:

welcome = welcome, {{user}}!
welcome = bienvenue, {{user}} !

Advanced usage

l10n arguments

You can specify a default value in JSON for any argument in the HTML document with the data-l10n-args attribute. In the last example, that would be:

<p data-l10n-id="welcome" data-l10n-args='{ "user": "your awesomeness" }'>Welcome!</p>

@import rules

If you don’t want to have all your locales in a single file or if you want to share strings between several pages, you can use CSS-like @import rules.

More information on the Language Selection page.


The following strings might be gramatically incorrect when n equals zero or one:

unread = You have {{n}} unread messages
unread = Vous avez {{n}} nouveaux messages

This can be solved by using the pre-defined plural() macro:

unreadMessages = {[ plural(n) ]}
unreadMessages[zero]  = You have no unread messages
unreadMessages[one]   = You have one unread message
unreadMessages[other] = You have {{n}} unread messages
unreadMessages = {[ plural(n) ]}
unreadMessages[zero]  = Vous n’avez pas de nouveau message
unreadMessages[one]   = Vous avez un nouveau message
unreadMessages[other] = Vous avez {{n}} nouveaux messages

Here, unreadMessages is an array and {[plural(n)]} points to the selected index.

plural() returns zero | one | two | few | many | other, depending on n and the current language, as specified in the Unicode rules. If one of these indexes isn’t found, the [other] index will be used by default.


By default, we currently assume that all strings are applied as textContent. However, you can modify the innerHTML property with a simple rule:

welcome.innerHTML = welcome, <strong>{{user}}</strong>!

Warning: this raises a few security questions that we haven’t addressed yet. In a future version we might:

  • sanitize the localized string before applying it as innerHTML (like in the PHP strip_tags method)
  • provide text-to-HTML methods (e.g. markdown) throught pseudo-properties, for example:
welcome#text = welcome, {{user}}!
welcome#html = welcome, <strong>{{user}}</strong>!
welcome#mark = welcome, **{{user}}**!

Further thoughts

Media queries

For mobile apps, here’s what I’d like to do:

<link rel="prefetch" type="application/l10n" href="data.ini" />
<link rel="prefetch" type="application/l10n" href="mobile.ini"
      media="screen and (max-width: 640px)" />

Multi-line strings

Multi-line and wrapped strings aren’t supported at the moment. The *.properties way to extend a string on several lines is to use a backslash at the end of line… but there could be sharper/easier ways to handle that.

YAML handles multi-line / wrapped strings nicely with the pipe and backslash operators, maybe we could reuse that in webL10n?

More structured syntax

There are cases where the entity has to be an array or a list (e.g. to handle plural rules), instead of a string. Currently we use the entity[key] notation but a more compact syntax could be supported as well.

Alternatively, we could use a JSON- or YAML-like file format to handle the whole structure in a more modern way.

Logical expressions

The Mozilla l20n/LOL project introduces the concept of “expression”, which can be used to address most grammatical rules or some very specific situations.

The plural() macro above could be easily defined as an expression:

plural(n) = { n == 0 ? 'zero' : (n == 1 ? 'one' : 'other') }

Browser support

Tested on Firefox, Chrome, Opera and Internet Explorer 6 to 10.


BSD/MIT/WTFPL license. Use at your own risk.


Client-side internationalization / localization library



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