Take the case of an English speaker who is perfect in English and decides to learn German from a language exchange partner who knows German perfectly and wants to improve his/her English. I think, maybe in the beginning it may be fine as you would learn the basics. But once you reach the intermediate level, how do you know if the other person is teaching you correct German or English (in this case) as he/she doesn't know the other language well and hence may be confused about the correct sentence structure.

It would be difficult for a German who is not good in English to translate a sentence structure from English to German, as he/she may not understand the correct sentence structure in English. So, in this case, even though he is perfect in German his translation would be wrong, as he may not know the exact meaning of a sentence in English.

Like in many Indian languages, we don't literally say, 'I am hungry'. Our sentence structure uses the word 'hunger' instead of 'hungry' and so is completely different.

While teaching Hindi/Bengali also, I have seen similar cases where students told me that they tried to learn these languages from someone whose English was not that good. When they talked to their Indian friends in Hindi or Bengali, after taking many classes from someone, their friends said that what they said didn't exactly mean what they wanted to say, but something similar.

In some cases their friends also got confused, as the words sometimes in English had a completely different meaning than what they have learnt. Like difference between, hungry and hunger. They used the word for hungry as khide (in Bengali) or bhookh (in Hindi) which actually means hunger. Our sentence structure for I AM HUNGRY is completely different.

In one case, it was only when they translated their Hindi friends in English, that their friends explained them that what she said in Hindi was not correct or the way she LEARNED was not correct. As her friends were living in America for a long time, their English was also good and so they could explain her well.