I don't know why exactly, but the Bulgarian language is unavailable at the game. When I opened the ufoai-bg_BG.po file from the game folder it says the language is Belarusian, not Bulgarian. Now there is neither Bulgarian, nor Belarusian language
( .
The only way to get it is to create the ufos folder with the custom .ufo file and my original ufoai-bg_BG.po file.