I understand Chinese is very difficult for one to learn well if one is not exposed to it at a very early age (less than 18 months.
That's pretty much true of all languages. It is pretty hard to reach accentless perfection if you learn the language as a secon language, say past about 5 or 7 years of age.
I also experienced this with Polish. My wife would say a simple word in Polish, and I would try - repeatedly and unsuccessfully - to reproduce the word.
It's interesting because the two sets of sounds (in Mandarin and Polish) are similar. The difference between alveolar, retroflex, and alveolar-palatal affricates and fricatives /s/, /ʂ/, /ts/, /tsʰ/. /tʂ/, /tʂʰ/, /tɕ/, /tɕʰ/ in Mandarin Chinese; in Polish, the difference between dental-alveolar, retroflex, and palatal affricates and fricatives /t͡s/, /d͡z/, /͡ʂ̠/, /d͡ʐ̠/, /t͡ɕ/, /d͡ʑ/. It's nothijng mysterious. Most non-native speakers have difficulty hearing and producing phonemes that fall outside of their native language's inventory. (It's interesting, most people complain about tones when learning Chinese.) when I was trying to learn Mandarin, I would practice with a Chinese friend, who was a native Cantonese speaker, and even I could recognize that he had a string Cantonese accent when speaking Mandarin. In isolation, one word at a time, say in a vocabulary list, it could cause confusion, but in context it's not as bad as getting the tones wrong. Some people just have a knack for mimicking sounds. Others don't. I still have a tough time hearing and producing the voiced and voiceless, aspirated and non-aspirated series of consonants in Sanskrit and Hindi: e.g., p, ph, b, bh, etc. Funny thing is I have some Indian friends who speak Tamil, and they have the same problem, though their Hindi is much better than mine. In the end, if you learn a language as an adult, you'll probably have trouble with phonology and syntax, although maybe one more than the other.
