I fullly support the idea to have the language preset to the region from which one is surfing wikipedia from. However, it is not obvious that the person reading wikipedia is comfortable reading in the langugage spoken in the region that they are in. For instance google have adopted this by having google.com load in swahili if you are in Kenya..not many people want to use google in Swahili and Wikipedia is no exception..
In fact I was in Tanzania last week and the guys were not comfortable doing google search in Swahili. We had to keep typing www.google.com/ncr to avoid the country redirect (No Country Redirect) NCR.
But again it would be good to have the language on the drop down menu as the closest,or the first to appear when a user opts to switch to another language which could be the one being spoken in their region.
regards
Stephen.