lucretius, you are right. The majority of EPUBs specify the language they are in - and Marvin detects it and knows it. The problem is invoking the dictionary in that language.
eVeNtInE, thanks. I'm glad you like it. Remember that the presets in the "custom button" configuration menu are just templates to get you started. I've written a post on how to set up menus for arbitrary languages
http://blog.marvinapp.com/post/49619863627