Ok, I had this problem, solved it while discussing it on the MTS2 IRC, but I thought that others might be interested as well.

The problem:
The "international" DVD edition only gave the choice to install in English oder Spanish, ignoring the locale set on the computer. The language packs for all other languages were there, however.

The solution:
- To change the language to German:
- Fire up regedit.exe
- Find the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\EA Games\The Sims 2\1.0
- Set "LanguageName" to the language you need. (Must be the same as in the TSData/Res/Locale directory of your Sims2 installation, e.g. "UK English" or "German")
- Set the "Language" to the corresponding number. This is trial and error, unless someone can post the order of the choices the installer SHOULD offer. US-English is "1", I guess Espagnol would be "2". German is "3".
- Save changes, start Sims2 and enjoy it in the language of your choice!