JIRA should follow this algoritm while trying to decide which language to display:

• User has a specific language selected on its profile use that one.
• If not, read the "Accept-Language" header and if that language doesn't match with the server default language, and we have a translation available for that language, select this one (this selection should NOT be persisted on the user profile).
• If none of the above, fallback to the Server default language setting.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
http://www.w3.org/International/questions/qa-accept-lang-locales

As most of our customers are from Germany, we've set up the 'Default Language' to german.
Registered user can change that in their profile if wanted. That's fine.

We've now also got a 'public' project, which is often visited by international users.
As being 'public' the visitors don't (usually) have an account, therefore they can't override the language defaults.

If we changed the Default Language from German to English now, all registered user would have to override the new language settings in their profile. This will probably cause a lot of hassle.

Feature Request:
It would be nice, if the Default Language offered something like "Use Browser-Settings" in addition to the set of given languages.