You should ask them in whatever language their Accept-Language HTTP header specifies. If there isn't one, then ok, maybe look at their IP. But it makes no sense to treat the IP as a more valuable source of information than the HTTP header that's there for exactly this purpose.
If there isn't one (which is rather improbable - AFAIK all the browsers have it set by default), then it should be either English or both English and the language assumed given the IP address.
