In web, login as admin in a site with MDL-57408 applied. Go to Side administration > Mobile app > Mobile features and add some custom strings in " Custom language strings ". Use at least 2 languages. For example: mm.sidemenu.mycourses|My Subjects|en mm.sidemenu.mycourses|Mis Materias|es mma.frontpage.sitehome|Home|en mma.frontpage.sitehome|Inicio|es In the app, login in that site and check that the custom strings are used. For example, if you used the example above you should see them in side menu. Change the language of the app and check that the language of the custom string changes too. Logout and login in a site without custom strings (either because they aren't supported or because the setting is empty). Check that the strings shown are the default ones again. Go to Side administration > Mobile app > Mobile features and add some INCORRECT custom strings in " Custom language strings ". For example: mm.sidemenu.mycourses|My Subjects|zy mm.sidemenu.mycourses|Mis Materias mma.frontpage.sitehomenotvalid|Home|en mma.frontpage.sitehome something not sense here mma.frontpage.sitehome|Inicio|es Check that only the correct language strings work (in this case the last one)