It is against Google's webmaster guidelines to put machine translated text where Googlebot can find it and index it. Google will penalize your site for auto-generated content for doing so.
Here is the excerpt from the guidelines:
Q: Can I use automated translations?
A: Yes, but they must be blocked from indexing with the “noindex” robots meta tag. We consider automated translations to be auto-generated content, so allowing them to be indexed would be a violation of our Webmaster Guidelines
Google's Matt Cutts says in this video:
A word of caution: There are some people who might think, "Oh, I'll just automatically translate all my content into 40 different languages. Now 40 different language markets can read what I have to say."
We are going to make clear (or I'm making clear right now) that our guidelines about auto-generated stuff can also apply to auto-translated text. If you are searching in French and you show up on a French page that has all be auto-translated, and it doesn't even read well at all, and no one has put any kind of human care into proofreading it or making sure that it flows, that is really a bad experience. So we would prefer that you maybe add a widget that says "translate into this language" or something like that and not just auto-translate into a bunch of languages and just throw that content up there.
John Mueller from Google says the same thing:
I just want to add a word of warning here -- using automated translation tools to directly create content for your site could be seen as creating auto-generated content, which would be against our Webmaster Guidelines. Instead of just taking the output of a program like Google Translate, I'd strongly recommend at least having it corrected before putting it online. While Googlebot may initially fall for some Spanish keywords in your text, your users are not going to appreciate content that has been automatically translated and published without a review. I love Google Translate, but if you publish the results and get them indexed without having them reviewed, you're not showing a lot of respect to your users...
If you want to know the top languages to target, Wikipedia maintains a page with the top languages on the internet by both number of websites using them and number of internet users.