The interface language in umbraco can be extended by editing small xml files and placing them in the /umbraco/config/lang folder.
The following languages are already available (besides the default english language):
German - download. Translated by Christian Langreiter
Italian - download. Translated by Stevie B. Gibson
Spanish - download. Translated by Gustavo Martinez
Dutch - download. Translated by Boris Leenaars
Danish - download. Translated by Niels Hartvig / umbraco
Swedish - download. Translated by Toft / SBBS
Portuguese/Brazil - download. Translated by George Justo / KnowledgeCenter
Norwegian - download. Translated by Thøger Amundsen / Doorway Consulting
To install a language pack, just copy it into the /umbraco/config/lang folder. Afterwards you can choose the language when editing a user in umbraco
If you've created a new language, please share it with the rest of the community. Just mail it along with your name and website, so you get the credit you deserve.