One of the more complicated topics in SEO is internationalization, geo, languages, and so on. It is really fascinating especially when you get into all the rules around spam.

John Mueller clarified in a Google Webmaster Help thread that you can use the original language of your reviews on a different language web page. For example, if your product reviews are in English, you can post them in English on your French version of that product page. In fact, Google wants you to do that over machine translating them and posting the machine translated version of them.

So when you post them in English on your French page, it is okay to add a button to translate them using Google Translate or something similar. This is assuming Google cannot access that machine translation and the user is aware it is a machine translating the original review which was written in English.

Here is the quote from what John send:

Forum discussion at Google Webmaster Help.