This is nice as a demo webapp, but I wouldn't rely on it. To sum up the other comments, you have two possibilities:
- If it shows the language as spoken (=official language) in a country, a significant proportion of the population might still not speak it, e.g. French in Canada (native French speakers mostly in Quebec) or Russian in Estonia (not wanting to learn the language of the former occupying power).
- Even if a language is not an official language in a country, you might still get lucky, e.g. there are a lot of Hungarians in Romania, you can get pretty far with German in places frequented by German tourists, not to mention English.
- If it shows the language as spoken (=official language) in a country, a significant proportion of the population might still not speak it, e.g. French in Canada (native French speakers mostly in Quebec) or Russian in Estonia (not wanting to learn the language of the former occupying power).
- Even if a language is not an official language in a country, you might still get lucky, e.g. there are a lot of Hungarians in Romania, you can get pretty far with German in places frequented by German tourists, not to mention English.