The differences between German and English versions are this:
- The binary is compiled differently (only affects retail though, FSO can deal with all of that stuff at runtime)
- The font is different, the German contains the extra characters that the English version lacks
- The voices are different
.. but that's about it. The tbls and missions are the same (even the English version has German string translations), and as far as I know all other data is the same.
To convert to a full English version all you really need to do is: get a copy of the English font files (in root_fs2.vp), make sure that the registry doesn't specify German as the game language (it doesn't have to be English, just delete the the key and the game will use the default), copy over the English voices.