I can see how things could get confusing with a language/culture barrier, as well as the fact that most omnivores don't dedicate a huge amount of headspace to vegetarianism.
Even on this thread, the definition of 'vegetarian' is getting confusing - eg:
Vegetarian = no food with a face.
Not difficult.
By that definition, surely most shellfish (mussels, scallops, oysters, clams, etc) would be vegetarian.
I do kind of feel that if you expect people to make a special effort to accommodate your dietary preferences, the least you can do is make a special effort to ensure that they fully understand what you can and cannot eat - even if that means specifically spelling out "I can't eat any meat (including poultry, ham and bacon), fish, shellfish, cheese, gelatin or any sauces made using any of the above, such as chicken stock." Yes it takes a bit longer to explain, but it also takes longer to meal plan/shop/cook for for a veggie if you're not used to it, so it goes both ways.