I assume they're not going to have some "Universal Translator", or have time to learn every language in advance, so they should probably know the language of wider communication wherever they're going to land .
(Of course, if they don't know in advance where they're going to land, then they should probably learn English, because then they'll at least be able to function in much of the world - but, if they do know, then they'll be best off learning the local language, whatever that is)