My buddy had the opposite version of this when we went to Japan. He was pretty good at conversational Japanese because that’s what was taught in his college courses, but most of our interactions were in a business context using keigo Japanese which is more polite and formal.

He struggled for a while, especially because we found that lot of Japanese folks would downplay their knowledge of English out of modesty. There were a number of times when our friend would struggle to find the right word and the person he was talking to would confirm the correct English word first before offering the Japanese variant.