I'd sort languages not by amount of speakers, but by territory size. English is spoken almost everywhere, so it's first language, Spanish and Arabic are spoken in a lot of countries and huge territory. Bengali is spoken by a lot of people but on very small territory, only one country and not so touristic and developed, so only a few people learn it.

Andrés that will be amazing challenge

Yes just because that don't mean people don't choice the spanish after the English or Mandarin... or don't mean spanish is a hardest languages...Before I speak English I had some problems for start be "fluent" before I allways said: OMG FRENCH IS SOOO EASY MUCH THAN ENGLISH, now I understand for me was the french more easy because is similar to portuguese because is latin have same level of difficulty and the english was "to easy" so was a little hard start talk or do phrases.

And I'm not bilingüe I'm polyglot but I'm not good enough in them yet

Omg Haru♥春 don't mix the natives maybe don't will like and stay mad with you

Haru♥春 I'm sure you will learn all very fast you speak 2 hard languages so I'm sure you will learn fast ^^