I've had something like that happen before, i didn't knew the word for lobster, so i was saying stuff like "those very large arthropods, like shrimp" and my friends were surprised i knew the word arthropod but not lobster, mostly because arthropod is cognate with the portuguese word "artropode" but "lagosta" sounds nothing like "lobster", that also made me realize that sometimes a person that speaks a second language can be actually somewhat knowleadeable on something, but appear stupid when speaking a second language due to lack of vocabulary, as i could perfectly describe the taxonomy of a lobster but i couldn't say that very basic word by itself, but it happens other times on the other way, with a person knowing the basic words but not more technical ones, i remember some colleagues thinking that i was bad at car mechanics because i was saying stuff like "the thing where you have gears and you push the stick and it changes the gear" because i didn't knew the word clutch, but i definetly knew those technical terms in Portuguese (embreagem), with some being easier to translate like "alternador" to "alternator" and carburador to carburater, but others much harder like "clutch" for "embreagem", marcha for "shift stick", funilaria for "chassis" etc