A message can begin in English and finish in Hindi, Gujarati or another language the participants share. The switch may be ordinary for those people, even if a writing tool treats it as something to correct.
Before standardizing a mixed-language message, ask what the change would remove. A familiar phrase may carry the rhythm of the relationship. Translating every word into formal English can make the reply feel less like the person who is sending it.
That does not mean mixed language is always the best choice. The recipient needs to understand the message, and some situations call for a shared working language or a clearer explanation of a term.
A useful assistant should distinguish between an unwanted error and an intentional style choice. It should also avoid inventing slang merely because it has identified a language or a region.
Try comparing two drafts: the original mixed-language version and a fully translated version. Which one preserves the intended warmth, clarity and relationship context? If a phrase is unclear, explain that phrase rather than automatically rewriting the entire message.
Personal voice includes choices about language. Those choices are best understood through the actual participants and their preferences, not through a rule that one standardized form is always more correct or more authentic.