It is one of the questions that I am asked frequently… Let me ask you a few questions that I hope will help finding your own answer:
1) Which language do you need to learn or will be useful if you learn?
2) If you do not have an answer to the question above, which culture or language are you interested in?
3) If no definitive answer for above questions, why are you asking which language to learn?
A few little things for you to consider:
1) You will spend lots of time and effort learning a language.
2) According to quick search on google, Mandarin (Chinese), English, and Spanish are the top 3 most spoken languages.
3) If you do not speak English, I would, personally, recommend you to learn English, simply because it is useful. However, if you have read this far and understood all what I have written here, then maybe it is good enough for you…
4) Languages that are close to the language that you already speak are easier to learn. For example, Spanish and Italian, and Japanese and Korean.
I cannot answer the question which language you should learn but I hope my comments above help finding your answer…