"takes offence without caring about the speakers' or writers' intention, or the conventions in their country"

There has to be a difference dependent on what the intention is though and there is a difference between not liking something and actually really taking offence. I don't think the word Yank is either negative or positive in British usage. It is just what many people use in a quite innocent manner thinking it pertains to all Americans. Fair dos once you know that it could potentially be disliked then it is best not to use it and I don't tend to use it myself but I do think the vast majority of Brits use it meaning no harm and not realising it may not be liked.

Many Scots are not too keen on being called Scotch but it is still seems to be often used in North America as the adjective. So if an American used the word Scotch then even though I maybe wouldn't particularly like it I would have no reason to actually take offence if it was used innocently.

So language is a minefield and we don't always know what might offend someone somewhere which makes it hard when we're communicating with people across the globe. So don't take offence if someone innocently uses a word you won't like - you just tell them in a nice way that it may be disliked by some. And if someone points out that something is potentially offensive then take note of it. why risk alienating people?