What language should I learn when traveling to Europe?

One language, English, is almost enough. Almost all countries use it. To say the least, there are many people in China, and there are many Chinese restaurants in European countries, large and small, so even if you can't speak a foreign language, there won't be much problem. It's just that if you can't speak English, it's meaningless to visit many places, just browsing.

French as a third language is ok, but it is not very common. It's good.