Do You Know the English Word for That?

If the word exists in English, you must use it in English, right? The examiner needs to see that you know the words for this and it could create confusion, right? I think about London and if you use the however you say London, if you're French and you say L'on or if you say ABBA like this student. That of course is going to bring down your vocabulary score. And it's especially sad if you do know the English word for that, but you didn't realize you need to use it.