So I find it to actually be a really helpful “barometer” of language skill. When I’m in France, if I go in a store and conduct s full conversation in French, I know my accent, word choice, and general language skill is good. If halfway through the exchange we switch to English, I know I either made an egregious language error or I started sounding like an American. If the conversation switched to English right away, I either made a critical language mistake OR I just happened across a very competent English speaker.