First and foremost, why one should use a desktop application if the goal to make reading comfortable for eyes? I use the desktop version as well as web and cloud versions (are there now two or one web-based version?) primarily to read technical books. That is, if I need search and read from several sources, I put on my Kindle on a cradle (a $1 cradle for paper books :)), open cloud and desktop version and it's better than navigate on a slow device.
Well, about the news: since several days ago, Kindle for PC support several language GUIs as well as several mono-language dictionaries. Gratis.
Okay, may you say, there is nothing useful if you use Kindle for PC only for technical reading. Yes, of course, no more languages for technical reading than English. However, there may open possibilities to take a time for foreign-language reading. Along with excellent Oxford and New American Oxford, and Duden, Amazon blessed us with Portuguese-to-Portuguese (Brazil), Spanish-to-Spanish, French-to-French and Italian-to-Italian dictionaries. Enjoy, learners, enjoy!