Sanskrit....the language of languages, mother of most other languages, oldest sacred sc<x>riptures, wonderful literature, an amazing, magical language.
Depends where you want to visit, and who you want to talk to. Russian is probably useful if you want to go there, therwise I'd choose Portuguese because you can use it in Portugal, Brazil, Angola and Madeira
I'd go for Japanese personally, but I feel like it's a bit tough to get started. Portuguese is very similar to Spanish so I'm sure that could make certain aspects of learning it easier.. After those, I'd go with Russian. I'm just personally not a fan of how German sounds though.
I love the Deutsch language but it's not easy , then again I've heard Japanese isn't either .
Japanese, i suggest it as i plan to learn it :3 if you are learning to travel, you cant top the culture of japan.