Problem with bulgarian language

The first time I installed OpenCart it worked well until I applied Bulgarian language as default for admin panel and shop.
It was givving me the following message: "Error: Could not load language bulgarian!".
I deleted the whole site and uploaded and insalled it again.
After that I gave Bulgarian to be default only for the shop. It works well.
But when I gave admin panel language default - Bulgarian - it gave me the same message - "Error: Could not load language bulgarian!".

Any help please!!!

*Edit*

Problem solved.
The latest Bulgarian language translation didn't have admin panel translated.
Now I downloaded from the previous version only the admin panel and everything is working fine now.
Just one question - how can I change manually between English and bulgarian language as default ( I mean without logging in as admin and changing it trought the menus?)
Where's that code?

*Edit*

Some functions aren't working with the old tranlation!

You will need to re-translate and update your Bulgarian translation and make it compatiblem with your version of OC.

I had to do the same for the Dutch version.