Welcome to the forum.
If all you want is to be able to type in german, then you can change the keyboard settings.
However, if what you are referring to is the MUIP (Multilanguage User Interface Pack) which will change the entire windows xp interface to another language, then you might be out of luck.
Unlike Vista, the MUIP or XP is not available in retail. There are two ways to get it:
1. You have a "volume license" for XP (for example like large coorporations)
2. You have a professional or universal subscription to MSDN
You can read more about it here:
1. FAQ about XP MUI packs
2. Dr. International on microsoft.com