I have Onyx Ereader and in my opinion Onyx firmware is very advanced and it can handle all languages.
It has 3 different readers: Pdfreader, FBReader and Cool-Reader
Pdf reader doesn't need any fonts. For other two readers you can install custom fonts as needed.
1) Support for non-roman characters: non-roman characters are supported but there are some small problems associated (they might be solved by now). The problem is/was that dictionary cannot select the word properly if the word contains non roman characters.
You can ask about this issue in Onyx forum.
2)Monolingual dictionaries: Onyx supports star-dict dictionaries only. All the dictionaries are here:
http://abloz.com/huzheng/stardict-dic/
Also, Android based readers are going to be launched very soon . I am waiting for them myself. I believe we will have almost no firmware problem or shortcomings in android platform.