Yes. ReadUps works with many languages. If a browser can read it, ReadUps should be able to display it. That means you can upload or import content in languages other than English, and comment in languages other than English. The table of contents loads for books organized for Western languages, though (left to right).
For uploaded content in languages other then English: the best way to get results you want for other languages is to have an .epub file that validates. Run it through the .epub validator and that should help if you're having problems. If it validates, it'll work in ReadUps.
The ReadUps.com service is located in the United Sates. Therefore, certain items (ex. the navigation) are in English. Let us know if you'd like to translate it to help localize it!