For the feature I was working on yesterday, @codinghorror wanted a rather complex sentence.

“There is 1 unread and 9 new topics remaining, or browse other topics in [category]”

This seemingly simple sentence was a royal nightmare to localize with our existing localization system. Think through all the permutations:

“There are 2 unread and 9 new topics remaining, or browse other topics in [category]”
“There are 2 unread and 1 new topic remaining, or browse other topics in [category]”
“There is 1 unread and 1 new topic remaining, or browse other topics in [category]”

Trouble with our current system was that you have no sane way of building these kind of sentences, see: http://stackoverflow.com/questions/16825932/clean-pattern-for-localizing-sentences-in-rails-i18n , you can only easily localize one count in a non compound sentence.

To alleviate this I introduce a new mechanism that is available (optionally) client side. The above sentence is localized using:

There {UNREAD, plural, 
   one {is 1 unread} 
   other {are # unread}
} and {NEW, plural, 
  one {1 new topic} 
  other {# new topics}} remaining, or browse other topics in {catLink}

The client localization file has a special rule, if a key ends with _MF it is interpreted as a MessageFormat message, then to access it on the client you use:

I18n.messageFormat("topic.read_more_in_category_MF", {"UNREAD": unreadTopics, "NEW": newTopics, catLink: opts.catLink})

You can see a few other examples here:

https://github.com/discourse/discourse/blob/master/spec/components/js_locale_helper_spec.rb

We do not plan at the moment to move to message format style localization everywhere, however it is nice to have this extra bit of flexibility that lets us generate interesting sentences.

On a technical note, this feature adds almost no weight to the client side JavaScript, all message format strings are pre-compiled into a JavaScript function with no external dependencies. The tricks used can be viewed here: https://github.com/discourse/discourse/blob/master/lib/js_locale_helper.rb

1 minute Message Format primer

f = "hello"
f() => "hello"

f = "hello {WORLD}"
f(WORLD: "world") => "hello world" 
f(WORLD: "other world") => "hello other world" 

f = "I have {HATS, plural, one {one hat} other {# hats}}"
f(HATS: 1) => "I have one hat"
f(HATS: 10) => "I have 10 hats" 

f = "I am a {GENDER, select, male {boy}, female {girl}}"
f(GENDER: "male") => "I am a boy"
f(GENDER: "female") => "I am a girl"

Our plan for now is to use this strategically, however it is worth noting that this gives more flexibility in localization, for example in czech, the plural form is rather interesting as @kuba could attest :

MessageFormat.locale.cs = function (n) {
  if (n == 1) {
    return 'one';
  }
  if (n == 2 || n == 3 || n == 4) {
    return 'few';
  }
  return 'other';
};

Message Format supports this fine, built in.

f = "I have {HATS, plural, one {one hat} other {# hats} few {# few hats}}"

Source: https://meta.discourse.org/t/message-format-support-for-localization/7035