Ok people, status update
So far we had EU based undub, with replaced sound files from the JP version, which caused problems with BGM, SFX and whatnot. After Majin3's hard work on coding and testing and a few of manywelps's ideas thrown in, we now have:
EU/US based undub, BUT
, with using the JP TLDAT file of which the content has been replace with EU/US content (except for the audio related files). We haven't tested this very far, but it seems everything is working, regarding sound (voice, bgm, sfx), English subtitles, English menus. There is a drawback though... There are JP text images left in e.g. the controls instructions.