The ability to switch the iOS dictionary language on-the-fly should be directly given to every iOS user system-wide. This shouldn't be the responsibility of developers of individual apps like Marvin.
For example, as I'm typing this forum post, I might wish to check, in the dictionary, one of the words I'm typing. The language I'm typing in could be
any language. I happen to be typing this particular post in English, but using the Slovak iOS keyboard, because it's not worth switching to the English iOS keyboard just for the sake of this post. The assumption that the active iOS dictionary language should depend on the language of the active keyboard, is plain silly.
There's no other way to put it: the dictionary feature in iOS is a disaster.
Only 6 languages are supported 6 years after the initial release of iOS. That is pitiful. The quality of the dictionaries themselves is as bad as can be: no internal search in them is possible; all hyperlinks in them are dead (!!!)
; you can't look up the definitions of words used in definitions themselves, etc.
The dictionary feature in iOS is an utter failure by Apple at this point.
It's great that Marvin, in version 1.5, allows us to use alternative, top-quality dictionaries of our own. (I recommend the third-party Lingvo app offering many dictionaries from top-of-the-line publishers.)