Hi!
This is not the right way to proceed. Renaming default.mo to bg_BG.mo hs no effect. bg_BG.mo must be generated by a relevant tool. You can use the plugin CodeStyling Localization to do this. It will allow you to translate your theme (no need to edit the .po file manually) and to generate the .mo file (which is the one read by WordPress, as WordPress totally ignores the .po file).
