To use another language:
- After installing and running Sunbird, exit Sunbird.
Note that Sunbird has to be run at least one time with administrator rights. Otherwise it isn't able to create the file chrome.rdf which is needed in the next step.
- Edit sunbird/chrome/chrome.rdf, find and replace
setting ab-CD to the language of your choice (provided that it is shipped with Sunbird)
- Remove XUL.mfl ( or XUL.mfasl ) from your profile directory
Next time you start Sunbird it will show up in that language.