You’re right, this is very likely browser based auto translation, which modifies the DOM and thus the text inside quoted posts in the composer, and there’s nothing to do about that since we don’t do any validation that the text inside quoted posts actually corresponds to parts of the quoted post.
FWIW, I’ve seen this exact issue in other forum software using inline replies based on markup like BB code and others.
We add a special CSS class to those modified quotes, so you could style those differently to help make users quickly tell faithful quotes from modified ones.