I am not sure which method to use in order to differentiate users. We can use geo-ip databases but I want to be sure that they are reliable. Maybe we can just let users select a location.
Any ideas?
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.
There is a discussion about this: http://www.w3.org/International/questions/qa-accept-lang-loc...
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.