Auri is an iOS application for reading books in ePub format and it has features for convenient translation of foreign words.
There are two ways for it:
Choose book in any other app, which supports file export (cloud storage, browser, other book reader, etc.), click "Open in..." and choose "Copy to Auri"
Via iTunes File Sharing (https://support.apple.com/en-us/HT201301)
Only ePub now. But there will be more formats in future.
Just touch unknown word while reading and you will see it's translation.
If you have internet connection you can translate word in almost any language via Glosbe.com API. Also there are offline dictionaries available (only English-Russian at this moment).
If you have a questions or suggestions, please contact me: email@example.com