Any Font Editor (*.fnt) for Precursors available?

I´ve decided to continue working on my german translation for precursors, but couldn´t solve the problem with german "umlauts" like dotted "a", "o" and "u".
Seems like the ingame fonts (main.fnt) can´t interprete these special keys, as there is always shown a russian sign instead.

Furthermore there seem to be no way to edit that font file. After trying a lot of font editors I figured out it must be a special format created by the develloppers. Changing these files and editing the XML files makes the game crash while loading. (Error: no valid signature -> new.fnt)

There is already translated german text from the creators of the game in the qre-editor containing german "umlauts". (ö,ä,ü,ß)
But changing the language from 1 to 5 (for german) in the game.ini file leads to another crash, while trying to load the game. (Error: Could not read stream #10 -> maybe missing german vids?)

Is there anything I have overseen? Is it possible to solve this problem? Unfortunatelly the sdk doesn´t contain a font editor.
So if there is anyone that could give me a hint, I would be very thankfull.

Thanks and best greetings from Germany
Pandur