ESL is, English second language.
So, who here fits that?

English is my second language, I wonder how many could tell?

I started Kindergarden as an ESL student... Spanish was my first language since my parents speak Spanish in the house all the time. Learn to read in spanish in Kindergarden it's quite easier to read in Spanish than English. I was an ESL student till about the 6th Grade... then They put me into a normal classroom.

I still know spanish even now... I plan to learn other languages too

Well i never took those classes (ESL) but english is my second language. I lived in mexico for 17 years...had to enter my senior year in high school, with regular classes, without really speaking english.

I heard that learning english as a second language is harder than learning most languages. Is this true(directed at people who know more than two languages)?

I am. Now I'm up to three languages though.

I can't remember, I learned English in Kindergarden... Years ago
I find grammar to be hard to get a hang of at times... As in some of the more complicated material

 Basic english is fairly easy to pick-up but it is incredibly difficult to master. This is what I have heard from everyone I know who learned it later in life and more than a few people who teach multiple languages.

I am ESL...

English is one of the easiest languages to pick up and one of the hardest to write. There are very limited forms of just about any verb and all you need is the smallest and most trivial changes or additions to form all the different forms of a noun/adjective. Writing/pronouncing is a whole story since English's "rules" are nonexistent or just pure shit.

ESL = English as a Second Language