Hiliter

This extension is pretty generic, it can highlight any words from internal configurable dictionaries, but, out-of-the-box, it comes with 6 carefully designed dictionaries, for all possible combinations between three East Slavonic languages (Russian, Belorusian, Ukrainian).

Those dictionaries were creatd by me, entension's author, by taking the wordlist of 5000 most frequent Russian words and then manually filtering out all word pairs that are similar (those three languages have ALOT of similar words, so the whole point of this tool is not just to have SOME highlighting, but to highlight exactly those words that are not trivial for any given language pair.

Extension settings page has full-fledged dictionary editor tool, that allows modifying existing dictionaries and creating new one from text file imports. For example, one can create, if one will, German-Dutch word list (having only nontrivial pairs, i.e., excluding similar pairs), or the same for Spanish-Portugese, etc.

I already submitted this extension to Google Chrome Webstore, but I believe that exactly Opera version makes sense for East Slavonic Languages because of large Opera user base for those regions.