You can provide the app menu in various languages. A sample ZIP file can be found in the Purple DS manager (see Configuration of Dynamic Content).

Find more information about app_menu.xml here: Individual App menu (app_menu.xml) . 
The menu of the app and of the web newsstand is not controlled by the language file in the app (more information in "Translation of Texts in Your App").

More information in the technical documentation.

The language of the visible menu is selected based on the device language settings. Purple DS app recognizes the general language settings. 

NOTE: The language settings of the browser set the app menus language based on the sub-folders within the "web" folder. If the language is available within the "default" (=app) folder

Country codes like "en" (= English) and "de" (= German / German) are derived from ISO 3166 ( https://en.wikipedia.org/wiki/ISO_3166-1 ). If the device is set to a language that is not explicitly listed, the app will default to the default settings.