They will initiate if they are unable to explain certain things clearer to you in English. I went into a random 7-11 in Shinjuku to ask for directions to Shinjuku Gyoen (we were not going in from the main gate). There were two staff manning the counter and they were talking to each other in very fluent Japanese. We asked them for directions in English,and one guy tried to explain in English but his accent was bad (he was a Japanese). Then the other guy tried to explain in English and his accent was actually familiar and we understood him better, but we still didnt quite get the directions he was saying, so he suddenly asked if we are from China, in mandarin. We were a bit taken aback because we didn't expect him to know mandarin. We replied back in mandarin that we are from Singapore, but since he now knows that we knew how to speak mandarin, he proceeded to explain in mandarin, which makes everything so much clearer lol.
He got the prc accent when speaking madarin but doesn't have the typical prc look. Extremely friendly and well mannered chap. I wish sg has more of such prcs in sg (but I noticed there are more well mannered prcs living among us nowadays already, so that's an improvement).