First: watch using the ` in place of '. I think on a Portuguese keyboard it's on the top left where we usually have the for "~" or "`" on English keyboards.
As far as "which" vs "what": in this sentence (in English) you'd probably only use "which" when given a choice of multiple population numbers. i.e. "Given 70, 70 thousand, or 70 million; which is the closest population of France?" But if you had no specific choices (it could be any number), you just ask what it is like the question given.