For practicality and profit reasons, I'd say Spanish since the majority of people that live in the US and can buy hentai that doesn't read English very well know how to read Spanish at a lot higher level than their English. That isn't to say, French, German, or Russian aren't in high demand - they're just not as demanded as Spanish, from what I can tell.
Though, if I had my choice out of the four, I'd pick Russian, since it's my ancestral language, and would give me a reason to learn it, lol.