Sometimes, also posts made by non-native English speakers can sound rude although they are not intended to be.

That's worth mentioning. Yes indeed non-native speakers often make a pig's ear of what they are trying to say. Even fluent in English Chinese can sound most severe if they simply, with kind intentions 'please sit down.' It's well worthwhile giving over whole lessons to intonation, indicating the attitudes and emotions; and the real meaning of the speaker. Take the word 'hello' this can mean many many different things ranging from 'I wish you weren't here' to the aspirations of the libertine. It's down to variations of pitch, and foreigners can make a mess of that. Hope this information is not boring. I'm a linguist.