If the geolocation api doesn't give anything - that probably means the user has chosen to configure it this way because they don't want to share their location. You should probably ask them in case you actually need to know where they are.

If one is not even allowed to guess with the user's IP the language this should be asked in, should the first question be some kind language selection form with just pictograms in it? :-)

True. But what language should you ask them in? Correctly assuming some details improves the UX for the many.

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.