This community made patching file patches your game files and turns your game into the German version completely. Download it here: https://mega.nz/folder/4IBlzR6I#U7K5yxL5JF1J_AWW1Yhbcg

You will find the JA2_DE.exe inside the archive. Extract it, launch it and specify the base location of your game, which contains the "ja2.exe" file. You can find it by opening in Steam the local game files when right-clicking on the game.
On Linux it will work too. Just Launch it with wine and set the game path on the Linux filesystem. For me my game path was "Z:\home\USERNAME\.local\share\Steam\steamapps\common\Jagged Alliance 2 Gold\" which corresponds in Linux to "/home/USERNAME/.local/share/Steam/steamapps/common/Jagged Alliance 2 Gold/"

Originally it was made for the GOG version and targeted the game version 1.12, however i have verified, that it works with the Steam version too on 1.12. I have not tested if it will work with community mod 1.13.

I am not the maker of this patch, so all credit is due to the maker of this community patcher. The original link of it is https://www.compiware-forum.de/downloads/file/461-jagged-alliance-2-german-patch/

Can i reverse it back to English?
No. The patcher will overwrite the game content files, so if you want a way to get your game back to English, you can either restore it from a previously made backup, let steam verify the files integrity (not sure if it works?) or simply uninstall and reinstall the game. Be sure to backup your save games first of all!

Is it legal?
If you have to ask, then you probably shouldn't do it. However as of 10 May 2025 I do not recall a single instance, where modding or patching a game has resulted in the police knocking on someone's front door.

I don't trust sketchy exe files!
Have it your way.

Here's what you can expect to see. I can say it's just like the game when I installed it from a German Disc.