Sites that offer reviews across countries have dealt with and resolved the issue of multilingual reviews in a pretty good way - show reviews in current language on top before showing reviews in other languages. http://www.tripadvisor.com.sg/Hotel_Review-g274887-d1185721-Reviews-Firstapartments_Inn_City_Center-Budapest_Central_Hungary.html
This has been brought up a few times in forums.
A patch is already available and has been submitted as a pull request https://github.com/opencart/opencart/pull/2700
This is default on sites I visit and should be in OC!