This question is being asked on behalf of @lvc, who had the concern triggered by a question where another user helpfully improved a machine translation.
Back in 2012, I gave this answer to a question: May we use non-English languages on this site?
Quote the original source (in the original language), but please also include an English translation. A rough translation is acceptable to start with. (If an automated translation tool makes a reasonably successful attempt, that is also acceptable.)
If you are bilingual and see a poor translation, please be bold and edit the post to improve the translation.
Should we permit user-translated text, or should we require links to support the translation (e.g. a third-party source, or machine translation), perhaps in addition to user-translated?