In this case the user was posting on local boards most of the time,if he just copied the content from other language and translate it into his language by translator then how to find that.
Sorry to bother,I am a noob at this.
Tough to find, but instead of using google.com or your country specific google, try to use google native to that poster.
Sometime back I found well written post but a single point was written in russian in whole English post. That line make me believe that it is google translation of russian language. I did not get anything from google.com but when I searched in google.ru I got my results.
If you really sure that somebody cross-copied then you can do the following:
1. Find the native language of that user.
2. Google translate the user post in native language.
3. Search in local google (native language) for the translated native words.
4. Open the search result and translate back to English.
5. If poster content and translated content matched, you find a culprit to be nailed.