A group of possibly Romanian construction workers live in my building, there's an apartment that's shared with a number of them. They haven't immigrated here, they just stay for some time doing the work, so I don't think that they have much to do with the surrounding culture and don't really have any need to learn the language. They seem to be polite people and always open doors for me when we're at the door at the same time (I don't have much to do with any of my neighbours, I just sometimes see them in the stairway). I've usually thanked them in Finnish, but I'm not sure if it's correct. I don't think that they speak it, but then I'm not sure they speak English either (I could thank them in German, Dutch and Swedish also but I don't think that they would help either). I'm not actually sure that they're Romanian, though it's the likeliest possibility so finding out the word in Romanian won't help me. I have no interest in getting to know them and asking them.
So, I think that thanking them in Finnish would be the best choice, as I think that even if you're in a country for some months you should pick up some common words (like I did in Holland) and even if they don't understand what I say they could maybe deduce it from the context. What do you think?