Interesting, good to know about these language-changes 🙂
The following translation-thing might be worth looking into as well. Currently email notifications are being sent in the language of the poster (using the user interface language) . Is this changing as well in this translation update?
Running international forums with multiple languages would be more interesting if this system works better (or more standardized). Now, for example, when you have enabled Dutch and English language pack on a forum and a Dutch person replies to an English person, the recipients (and subscribers) will receive a Dutch email about the new post (or mention). The same is happening the way around.
$locale value in the Translator, but as far as I know this isn't being used in any extension to look at the reciever language (for example, in the UserMentionedBlueprint.php#L68 file for mentiones and this file in subscriptions).
I think it would be great to have something like this properly standardized (or having a different solution) and built into all core extensions as well. Maybe I am missing a topic and it was addressed elsewhere, but I couldn't find anything about this yet 😛