If you like flashing custom ROMs, sometimes it can be difficult to find one with the required language keyboard. On the more recent HTC Windows Mobile devices, the EzInput suite of keyboards has been popular and quite widely customised.
XDA forum member deckoff has helpfully modified the French keyboard to adapt to a phonetic Bulgarian layout as well as instructions on how it was done. If you want to have a go yourself, you’ll need to replace two files in the /Windows/folder which are original ROM files. Because they are original ROM files, you’ll need an app like Resco file manager.
Note that the dev has only modified the full QWERTY layouts for portrait and landscape keyboards. The Compact and Phone are not modified and hence will have the default French layout. deckoff however is keen to learn more about modding the EzInput suite of keyboards and hence any knowledge is welcome.
To download the mod and for further info, head on over to the modification thread.