@NaturalMineralWater it’s a very hard question, I honestly don’t know. I like Greek, because it’s one of the basic Indo-European languages, and it’s quite useful and helps you gain insight into other languages (English for sure). You can see the roots of words, and understand why the spelling is sometimes weird.
For similar reasons, I’d like to speak Russian or preferably Lithuanian, which I think is an amazing language (and the oldest modern language in Europe). I speak a little Czech and Polish, but not enough.
I love the sound of Portuguese, which belongs to another useful group or languages. I tried to learn Latin at school. It was the only lesson I was willing to study hard for, but unfortunately both my teachers happened to be crap. I guess Italian or Spanish would also be useful (I speak none of those languages, but understand a little due to my good knowledge of Greek and English).
But I think if I had to pick one, it would be Dutch. Dutch is amazing if you think about it. It’s like a mule. It is half German, half English. If you speak both languages well enough, you can easily understand Dutch, at least written. I can read a Dutch newspaper and tell you what it says, even though my German is not perfect. Similarly, someone who speaks perfect Dutch could easily learn both German and English, or at least understand both languages without major problems.
So I guess I’d pick Dutch in this case. Especially if you think of the practical implications (German and English are a lot more useful in the modern world than Greek or Albanian for example).
I have not mentioned any non-European languages, as I know too little about them to form an opinion. I did two years of Arabic at university, and became quite fluent at it (I was dating a Lebanese girl at the time too). I find the grammar and spelling fascinating, but it’s an entirely different kettle of fish, and I’ve decided to stick to Indo-european languages for now.