depends on what is your mother tougue.
if your mother tougue is English, you should learn similar languages like spanish, italian,
if your mother tongue is Japanese, it will be easier for you to learn Chinese or Korean.
but to learn a language really comes from interest. Interest is your best teacher and can make it a lot easier.
but if you looking for a language to learn to be more competitive for your own career, your future , or for your kids' to be more competitive in the future , you should learn Mandarin. because china's economy booming now. ppl are involved in business relationship more and more.