I think this covers everything, but don't shoot me if I missed something!!
The following files contain messages that are either sent via 'error_handler', or 'MessageBox':
audio.c, decode.c, encode.c, encthread.c, main.c, misc.c and wave_out.c.
decode.c, encode.c and main.c write text to a variable named 'approxBRCaption' that you may want to translate.
Additionally, main.c will need quite a lot of attention. Text you may wish to translate can be found at line 100 - *noiseShaping, line 105 - *wavFormat, line 110 - *GenreCaption, line 747 - 'DrawText'. Also, the CALLBACK routines contain many dynamic messages that will need translating.
I think that all of them!! Good luck.