I would love it if Duolingo would apply mouse-over translations for ALL words on the site. So, if someone typed a comment in Spanish, I would like to be able to mouse-over a word and see the English, and vice-versa for English to Spanish. I guess you would have to set which language you want to translate to in settings or depending on which section of Duolingo you are in.
If you use chrome, I recommend the TransOver extension . There is something similar for firefox , possibly hovertranslate or something. It doesn't always guess the right language for a word, if it is in multiple languages for example, but it is plenty good enough to rely on if you are already familiar with the language you are reading.