for chinese, we can hear each other, but maybe we can not understand each other, because we can't know the real meaning behind our sentences if we grow up in families with very different culture. Then a Chinese without good education experience can not give out a well orgnized paper. even the educated, can't work out a satisfied one.
chinese from the very south and the very north with their own dialects can not understand each other at all.
for foreigners, at beginning, the four tones and pin yin like zh-ch-sh, j-q-x, z-c-s are difficult.
Then with the time of learning, one charactor with several meanings, one pronounciation with some different charactors, sayings, proverbs and useful expressions are difficult.