Hi niffiwan
If I remember correctly, FSO uses the file checksum of font01.vf to determine what language to use with English being the default if it gets a checksum it doesn't recognise. Given that checksums are (for most intents) unique to a specific file I don't believe you'll be able to use this method to automatically select German unless you can modify the source code for WCS to add the relevant checksum for it to recognise.
WCS uses MD5/CRC32/whatever on font01.vf to toggle the language? Yes, then i have no chance to build my own german font. ^^
But after my posting yesterday i have tested some things and the good news (for me) is, i don't need to change the font01.vf, this font is used for the GUI and game engine (what i can see) and this font is okay for our translation. What i need to change is the font for the Fiction Viewer, currently font04.vf. The problem is here: in the mission file i can select the font for the FV, but it seems only the fonts (font01-font04) in the original WCS .VP files is recognized. A font in our mod directory will ignored by the FV and i have no idea why. Any ideas?
The point is, font04 is working properly, but our german "umlaute" is not shown optimal, especially the a-umlaut and o-umlaut are realy bad, so that's the cause why i need to change this font a bit. On the other side: this is not our top priority, so if we found no solution for that, well.... never mind. ^^
The language can also be set by adding an entry to the registry (on Windows) or in fs2open.ini (for Linux/OSX), this may be the better option for selecting the language in your case. The problem is, I don't know if WCS has changed any of this since they forked and I can't check their source code, so I don't really know if this advice is valid for WCS 
Generally you are right, this
should be the better solution, but it isn't.

We had big problems with our translation by using the registry to toggle the language. Long long story... i had trouble with this issue more than a year. Short: Using the reg value to toggle WCS into german language is a really bad thing! Only if we
delete this reg value completely
and if we use the special font01.vf all works as designated. To find out this has cost many time and many nerves.
