In the administration panel>configuration, set the default language to Bulgarian.
As I said before, I already did that.
The setting there will determine the default setting for guests and new users.
Well, that is the strange thing. The language for guests was still English.
All existing users will need to change this for themselves in their profiles.
It is OK right now. After I changed the personal setting of the admin user (the only user), everything went right. I think that's a bug and I will report it when I have time.