Russian is quite hard if you don't speak any slavic languages, even for me it's annoying to learn and I don't really like the sound of that language (which is subjective ofc). I'd recommend learning Spanish or French, whichever you prefer (a professor at my school who speaks both actually told me that French is more useful in business and also a little bit easier, don't know if it's true though). Scandinavian languages are really easy, I wouldn't really recommend learning Danish because their accent is... very weird, but both Norwegian and Swedish are awesome. I think I like Norwegian a little bit more. If you really really like languages, then I'd also recommend learning less popular ones like Afrikaans (very easy for English speakers), Arabic (hard, but worth it in business) or Portuguese (not very hard).
It takes up a LOT of time to learn a language, but make sure to make a routine and stick with it, practice as much as you can. I'd also recommend buying books about the language that you're learning.
Duolingo is a good starting point, but it won't really make you fluent.
Just my two cents :D