tedster - 1:00 am on Nov 16, 2010 (gmt 0)
A manual review can be triggered when the translated URLs start to rank on important queries. If the pages don't ever rank - then it doesn't really matter one way or another. But pon review, it doesn't take long for a human to see the difference.
Google also monitors the number of impressions for any domain, as well as sudden jumps in the number of URLs found. I think a manual review might also get thrown in those situations.