English has one word for "shell" while French has two: coquille and coquillage. Coquillage refers to an empty seashell, the object you pick up on the beach. Coquille refers to the shell of an egg or a nut or a live shellfish. When I was a teenager staying in France for a summer, I always seem to use the wrong one. Worse, the word for poppy, coquelicot, felt so similar in my mouth that often used that, too, resulting in lots of laughter from my French friends.
To confuse matters further, coques, a similar word, is a ship's hull, though I have also seen it translated in a recipe as cockles.
In English, I think the group of words which give ESL learners the most difficult time are all those ending in -ough since that has 6 different pronunciations. Worst, are those which which start with "t": tough, trough, though, through, thorough, thought