The Apps will use your OS primary language, both in iOS and OS X. Please note that each app is localised in a different set of languages, if your language is not available it'll be defaulted to English.

Change Language Settings on iOS

Go to Settings -> General -> Language and Region.

Change Language Settings on OS X 

Go to System Preferences -> Language and Region.

At the moment is not possible to change the language on an app by app basis, unless there is the specific option in the Settings of that app. This may change in the future.