Faterson, Marvin is already reading the book language so that shouldn't be too much of a problem.
Regarding dictionaries, unless I go all the way and use custom dictionaries, I'll have to depend on iOS services. I cannot support custom dictionaries yet because of the licensing costs and the freely available ones I experimented with have some issues with iOS. The iOS API doesn't let you switch dictionary languages directly but so far I managed to emulate this behaviour (somewhat). I'm still developing this area so I might have good news in the future.
As a side note, I managed to get in touch with the guys at Abbyy to make Marvin integrate with their multi-language dictionary better.