I have that problem with the continental scandinavian languages - I know all of them almost on intermediate level, but I know I have to pick one if I want to be fluent and stop mixing up, and I personally can't be fluent in all of them. I just don't know how to decide.
I strongly agree with lingoboy. I often don't find logical reasons for why a language interests me. Sometimes I meet a person, discover a new band, find out about a cultural trait..... and then the language starts interesting me and then I usually find the sound of the language so fascinating and it becomes a vicous circle
On the other hand there are languages I should learn for obvious reasons of my career choice, but I just can't come up with the motivation, I can't find the spark that lights the interest (that's Russian for me...
I am learning Welsh because it started interesting me when I first saw a few sentences in Welsh written in my 7th grage English coursebook
And I'm really happy that there are a lot of learning opportunities on the web and a great choice of affordable month-long summer language course in Wales (I hope to finally take my first one next summer). I got sucked into it
If you prefer a more practical approach, I'd also recommend you to look at the number and percentages of speakers, because of course that affects your chances of actually using the language
And you could take a look at the learning materials online and what you can get your hands on in Oulu. I decided not to use anything with german as a source language, because I think it will be more useful for me to know the english terms and explanations when I'm studying in Wales.
A friend of mine told me lately that if he would choose to learn a celtic language, he would learn Manx, because he would like to be part of the revival community and he would like the opportunity to invent words and introduce them into a language so that they'd actually be used, which of course works only with small speaker communities^^
Edit: Help, sorry for my terrible English. I'll leave it like that now, but I just saw my own violation of the rule that we learned about at school: "If + would = Satz kaputt"
Hopefully my old english teacher won't find out.