I wouldn't do it. I have no 100% proof that it causes ranking problems, however Matt Cutts has talked about using auto-translated content in a very negative way. This is especially true when each page has its own URL.
The biggest problem is that auto-translation is still very weak. If you essentially multiply your site's size by four, and 75% of those pages are poor grammar, then I think you could be risking a Panda problem.
If you really feel your customers need this, I'd say keep those auto-translated URLs out of the index, and preferably not crawled at all using a robots.txt Disallow rule.