Do you mean I can only choose one?
I’m fluent in English. Je ne comprends qu’un tout petit peu le français. I can pick up a little Spanish, but I’m not at all confident in my ability to understand the language. I can say a few sentences in Japanese, but haven’t attempted to learn how to read any of it. I know a handful of German, too.
I don’t know that I could choose one language… maybe one that is further from a Romance or Germanic language. Maybe Russian or Arabic.
If I could, I’d stick a Babel fish in my ear so fast… I love languages.