"Mind your own business" is an idiom that means "don't interfere in other people's affairs and focus on your own". It's commonly used to tell someone to stop being nosy or asking about something that doesn't concern them. In this context, it's used to imply that the person asking the question should stop being curious or meddling into matters that are not their concern.
The phrase "how to say it in English?" is a straightforward question asking for the English translation or equivalent of the Chinese phrase "关你屁事". It implies that the person wants to know the English expression or wording for that particular phrase.
Therefore, "Mind your own business, how to say it in English?" is a direct and slightly sarcastic response to someone who is prying or being nosy and also wants to know the English translation of the phrase "关你屁事".