here is the source you need for translating.
The files in the 'translation' tree + file 'msp_prefs' contain all the strings, on every line there is an English phrase first, followed by TAB. Please place your translation after the TAB.
In some files (msp_prefs and *.properties), the format is like this: 'key=text'.
Just replace the English text with appropriate Bulgarian translation in these files, don't touch the keys.
The graphics can be found in the 'ui' directory. Use your favourite graphics editor to put Bulgarian texts where desired.
I will also need a Bulgarian keyboard layout for Physkeyb
As for building the update bundle, I can do it for you if you want, or, if you are proficient in Linux, I can offer you my build system. But be warned that it is a little bit rough around the edges.