Can I change the LANGUAGE?

Can I change the LANGUAGE? And how?

Wotlk or MoP?
and which language?

If you want to change the language of your client, you can do that... in most cases. Just go to the page for your mother tongue:

https://i.imgur.com/F4bV8YP.png

and try to find a thread, that mentions the procedure to change the ingame language to your local one. I have seen some players posting their professions in German, French, Russian and Spanish, so there is certainly a way to use them. Though, I haven't seen Bulgarians or Romanians playing WoW in their respective language, so I can't guarantee, that you will be able to change the client's language to your local one, if it's not widespread.