Send a translated message to the subscriber

I have patched message notify such that it supports translated messages:
https://www.drupal.org/node/2177929#comment-9150847 →

But for this to work in combination with message subscribe we need a patch here as well.

Next comment contains the patch. I'm not sure if my method is sufficient, it might be better to let message_subscribe_get_subscribers return a language code such that other modules can modify the language.