Set language in constructor of \RestfulBase 

(follow-up of #318)
Thanks for the test!

I think we should trim it down a bit, though. I would expect a smaller test case, where we simply assert a single/ multi value fields for 2 different languages - so the test would be really focused on the functionality, easier to maintain, and easier to follow.

Also, I do believe entity translation shouldn't be needed. For example, the Message module uses only local to provide translatable fields.