How about explaining the situation to your manager and proposing a discussion among your manager and all the members of your team to agree on some guidelines fo

How about explaining the situation to your manager and proposing a discussion among your manager and all the members of your team to agree on some guidelines for language use within the team? In my experience in multilingual situations, people tend to shift to their first language just because it’s easier and feels more natural; it can also feel awkward to begin a conversation in one language and continue it in another. People rarely use their first language to intentionally exclude others from the conversation, but they often forget that others might justifiably feel excluded. A nonconfrontational discussion might provide a sufficient reminder to the Mandarin speakers to use English when you are around. You might also find out whether the Mandarin speakers on your team have any issues with English use in your team that you might not be aware of, such as the native English speakers talking too quickly, using slang they don’t understand, etc.