To change the ingame language in Creatures 3, you have to edit the "language.cfg" file in your Creatures 3 installation directory. it should look like this:
Language en-GB
LanguageCLibrary english-uk
Change the above lines according to your needs.
To change the language to German for example:
Language de-DE
LanguageCLibrary german-de
Enjoy!

If you do this you may have to download the fixed Creator-de.agents file from AmberCreatures ([url=]http://www.webpetz.com/creatures/updatesce.php[/url]), because otherwise you won't be able to see all available agents in the Creator in the german version of the game.