Wrong language detection due to email signatures
It’s relatively common for email signatures to interfere with language detection of customer requests.
To prevent this from happening, it’s possible to remove email signatures from the request, before submitting it to the translation engine.
In order to do this:
Navigate to Project Settings > Apps > Language Translation
Scroll to the bottom and click on “Additional Settings“
Disable the option to use Email Signatures for language detection
This setting works only when your Jira Service Management has been configured to hide email signatures. See: Hide email signatures from the issue view and portals

Note that this option can be used in combination with the restriction on the number of characters used for language detection.