I finally got around to doing my day skipper last year and on the course was a Russian who spoke English perfectly fluently but struggled with the technical terms that the rest of us took for granted. It probably doesn't help that sailing is one of those sports where the difference in terminology between British and American English is sometimes significant, for example I don't think the term 'kicking strap' is used in the US.
The knots I use most often even outside of sailing are the bowline and the round turn and two half hitches.