I speak quite a few languages, and most of them badly, good enough for communications needs but not precise enough to win me any literary prizes. If we were having this discussion in Dutch I would definitely not pick on you for using words that may not be precise enough or that might be offensive to you simply because I'd assume that you must have learned your Dutch from other people speaking it and using it in your vicinity, coupled with media such as movies, books and so on.

I've seen the word 'retarded' used in the exact way I used it above in many places and I've checked the dictionary and it has a simply secondary definition ('Dumb, foolish') which is roughly what I intended to convey.

Since giving offense is not one of my pet hobbies I edited my comment but had the situation been reversed I would have definitely afforded you more leeway.

To me 'retarded' did not seem rude or derogatory until you pointed it out, judging by the number of other people that are actually native speakers that use it in that exact same way it seems to me that you have your work cut out for you.