Which one of these languages should i learn?

It really depends on your ability to adapt. Do you have a high tolerance for ambiguity? According to the Department of Defense standards of languages...Chinese, Korean, and Arabic are the top 3 category 4 languages as far as difficulty goes. The rest are supposedly less difficult. English is a category 5 language...it's considered more difficult because it changes too quickly. More people in the world can speak Mandarin Chinese than any other language...it's not going to be easy if you want to compare it to what you already know. I speak it courtesy of Uncle Sam...our motto at Defense Language Institute was 'I speak Chinese so you don't have to!' Once your mind begins to expand with your first, second language...then you can pick up other similar languages...the younger you are, the easier. I learned Mandarin in my mid 20's...lucky for me, I already knew three other Chinese dialects before that, so I was able to retain more.