I'm a doctor, not a mechanic
The Unreal engine supports language localizations (or localisations for the English amongst us) in a very straightforward and easy-to-use way. If you declare a string variable using the localized keyword, the engine will automatically look it up in the selected language's localization file, potentially falling back to the international language localization file first and the value declared in the default properties at last.
- The file extension and language code for German is .det, not .de. (Similar modifications apply to the other language codes; it's also .est, not .esp, and so on → the last character always is a t.)
- To switch to a different language, open UnrealTournament.ini and replace the language code in the line "Language=int" by the code you wish to use. If the localization file for the Core package is available and the Language value in the [Language] section is set you can also select the language in UnrealEd Advanced Options -> Drivers -> Language.