I'm amazed with Marvin! Really amazed.
I wrote a post time ago about foreign hyphenation... and it recognizes language in epub and does separation conform each language, at least English and Spanish one. For Spanish, after reading 4 full books with Marvin I've not found any word wrong separated.
However, I think you can improve dictionary support. That is, my iPad is in Spanish and it is capable to find Spanish words. However, I read English books and would like to use English dictionary (does not matter if it eng-eng or a eng-spa). Easily said, as native Spanish talking I do not need Spanish dictionary.
Have you a custom dictionary installation or customization in your roadmap?
PS: A workaround is put iPad in English, of course.