Don't mix language and location. I might be in Spain but I might want to chat in Finnish.

For language, yes, allow the user to set it. However, you can probably use the Accept-Language http header to get the right language for most users.

Yes, we may try the Accept-Language. But I need to make sure that it is reliable.

There is a discussion about this: http://www.w3.org/International/questions/qa-accept-lang-loc...

Why not try a combination?

As long as the user can ultimately decide on their own, you can try a best guess for them. Just make it easy to switch and let the user know first thing.

Yes, I think that would be the best solution.

As a follow-up I ran into the exact same issue as above. I think the language-matching is really a must for an app like this to ensure users have a good experience their first time.