The language area of study is one of the most complex ones that the human brain ever produced. This is obvious from the philological point of view but we should understand that properties and options connected with languages are an extremely complicated area when it comes to SEO as well. As much as this is fascinating, it can get really frustrating when you connect all internationalization, language and geo options to all the spam rules that we need to take in consideration.
In a Google Webmaster Help thread, John Mueller from Google did clarify that using the original language of reviews on a web page in a different language is just fine. Not only that, but Google encourages all the users to use this exact method instead of using machine translation and posting the machine generated text. So, if the reviews of your product are in French, you can easily post these French reviews on your German web page version and this will be no problem at all. Here is another aspect that was mentioned by John, which should be considered:
I’d recommend making sure that you have the hreflang connection between these pages set up properly, so that we show the appropriate version in search.
Another good advice is that even if you are using computer translations you should not index them since they are automatically generated content and could be considered as spam.