Drupal.t() and language code I want to write some JavaScript code for my project. I need to use Drupal.t(), but it seems that Drupal.t() only accepts English, and I don't want to write my JavaScript in English.

Is there any way to inform Drupal.t() that my string is not in English, just like t() that let you specify the 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.