Drupal.t() and language code
Nope, not really. Drupal is built from ground up in English and that's the way you need to build your modules too. Some parts of the code tries to enable you provide default texts in another languages, but it's a disaster if you decide to change default language later.

It is possible to built functionality like you need in a custom module, it is even possible someone did, but I don't know of any reliable solution.