@Jay_ In what context do you have to speak to them? Because less than half of the French population speak English fluently.
They could probably say "Hello", but wouldn't be able to hold a conversation.
It really depends on you whether you have enough effort to learn the new language or not, choose the one that you really like and will most likely make the use of it after you learned it. I am currently learning Japanese which I really like it and the way of making you learn it better is exactly what @Duffy said, I just watch anime or listen to Japanese music which is a really a effective ways of learning language. But what most important is your effort and dedication to learn it.
@Jay_ Understandable, but even if you stay in the UK, it's always good to know, Moroccans speak French as a 2nd language usually.
It's always good to know, as doing business with French companies will go much easier if you speak their language.
@Jay_ Written Danish is pretty similar to Norwegian but I don't think Swedes or Norwegians can understand a word of what they are saying, Swedes can understand norwegian though.