Regarding the upcoming integration of German/Spanish/French dictionaries in the eagerly awaited Marvin 1.2 version in January, please make it possible (if it can be done) to switch the dictionary not just as an app-wide setting, but also on-the-fly. When you read Tolstoy's War and Peace
-- no matter if in the Russian original or in an English translation -- you have whole sentences in French included in the novel. Readers might wish to look up words from those sentences, and changing the dictionary for the entire Marvin app just because of that, wouldn't be efficient.
Again, the Kindle app can serve as a role model when it comes to dictionary integration. Whenever a dictionary definition pops up in the Kindle app (and it's great
that it pops up without your having to press any "Define" button -- that's a big time-saver!), there's also a small "i" button next to the definition. You can press that, and this allows you to change the dictionary language on-the-fly. Please make a similar arrangement in Marvin, if possible.
(The Kindle app is well ahead of iOS in that it already supports 10 dictionary languages. Russian as a big language in particular is sorely missing in the list for both Kindle and iOS.)
among metadata editable in Marvin, you might include a "Book Language" field. Even if Marvin weren't able to detect a book's language automatically, users might set the book language manually. That way, if you opened an English book in Marvin, English dictionary definitions would pop up; for French books, French definitions, etc. The dictionary selection would become automated. I don't know about other Marvin users, but I frequently read books in various languages every day, and having to select the appropriate dictionary language manually in Marvin every time won't be optimal. Even if the dictionary selection were automated in Marvin, the option to change dictionaries on-the-fly would still be useful for looking up (say) those French words embedded in books written in a different language.