From what I recall from previous posts in this thread from users of other language versions (like the one at the top of this page), you can just rename the english text folder to your current language (German I belive). You will then have a mix of german and english text (the english parts being the new and changed parts from killap) and german voices, but at least you'll have a working game.
If you are seriously interested in translating, the nessasary english files for translating are all in that text folder.
\Fallout2\data\Text\English
Theres about 700 differant dialog files totalling about 3 megabytes (and the combat messages which are nearly another megabyte), so its a pretty big job.
From what I can tell you can edit the .msg files with notepad.
To give you an idea of the size of the job you are getting yourself into, I belive the english Bible is close to that size as a pure text file. Although the dialog files have a bunch of # comments, and {} marks, etc that you wouldn't need to translate, so its not quite as much as all that.
I'm sure all the german fans would love you if you do it though
Edit: You could probably extract the original german files in your copy of the critter.dat and master.dat files and use some of the stuff thats already translated in there, but you'd have to compare with what was changed in killap's stuff. Not sure which way would be better in terms of the amount of work.