Qube is right, MJM files are basically PKWARE DCL files (00 06 is a valid PKWARE DCL id) with MJM1 inserted before.
Gods PC version has files like this (without the MJM header).
I've downloaded & compiled dynamite (which allows to unpack those files) using Cygwin (not mingw: it crashes). I guess VC++ would have been ok.
I'm now able to DECRUNCH FILES FROM GODS & CADAVER PC oh my!! So big thanks to Qube for the MJM link. The compression format has been reverse engineered in 2001 (fortunately for us!!).
For Gods files:
dynamite PLEV1A.DAT LEV1A.DAT
For Cadaver MJM1 files
dynamite ENG1.LNG ENG1.LNG_unp 4
the Gods map files seem to be good. The Cadaver language files do not contain ASCII, tough luck.
BUT other files contain gfx data (i see swords, switches,... using gfxrip). But I'm afraid that the gfx data does not have the same size so it's tough to rip using gfxrip.
I'm putting the dynamite unpacker source & cygwin exe (needs cygwin1.dll, but I include it) on my website: