Speed and slang.
I too have English as my first language too, but there are three other languages I had to know to some degree, as those were what my parents spoke: Hindi, Kannada and Tulu (sp?)
India has 24 different countries, and unlike the US with like 7 different versions of English, there are 24 DIFFERENT languages. There are even more breakups within a country. Example, in the southern Indian state of Karnataka, my mom is from Udupi and she spoke Kannada (pronounced like kuh-na-da), and my dad was from another part and he spoke Tulu.
By the time I was old enough to take a trip back there, my main problem in learning the language was the speed in which they spoke and any at all slang they used. One should remember, when learning another language, your learn words, spelling, grammar, sentence structure, etc. but not so much social interaction mainly because we are all learning this new language with people who speak our own and we are not always speaking what we should.
More importantly, and this is with respect to my folks and any teachers, there were the differences in interpretation. By this I mean, say there was a word you wanted to have translated, what if they (your mom/dad/teacher) did not know what you were talking about. That becomes a problem. In my dorm, we have 3 other suitemates from France and they more or less have a situation when trying to figure things like, " what do you call this" and they hold up a bath towel and rub their back.
I hope this helps, I have a bit more, but I'm starting to get hunger and I am going to go to the "Hanger".
The meaning of life is curiosity; we were put on this planet to explore opportunities.