French, Spanish, Italian, and probably Arabic. Most of the countries and provinces worldwide have multiple common languages, so half the time it depends where you're going for travel.
Either Spanish or French, depending on where you are going. The only problem is that, especially with Spanish, the language varies wherever you are going. For example in South America the language is based on Spanish but it is not exactly the same. French on the otherhand is spoken in some African countries, France, some European countries (as a second language, but you will find that most speak English as well) and Canada, however, once again the language can differ. If you are talking about using it for business purposes Chinese Mandarin is also a very useful language to know, but I wouldn't worry too much as many people are fluent in English across the world, but if you wanted to learn a language I would recommend Spansih or French, depending on what you are doing and where you are going.