This issue only needs to be tested in 1 device. After changing values in the 'Custom language strings', please click to Synchronize the site in the app and then restart it to make sure the lang strings are currently fetched and loaded. Login in a Moodle site as admin and go to Site administration > Mobile features. In 'Custom language strings', configure the following cases: Configure a string for the child language but not for the parent language. E.g. a string for 'es-mx' but not for 'es'. Using an inherited string, configure a custom string only for the parent language. E.g. in es-mx the 'core.more' string is inherited (it's the title displayed in the 'More' tab), in that case you should configure the 'core.more' string for the 'es' language. Using a string that is different in the child language, configure a custom string only for the parent language. E.g. the string 'core.settings.appsettings' is different in 'es-mx' than it is in 'es'. This string is displayed im the More menu, it's the option to open the app settings (Configuración de la aplicación).  Using an inherited string, configure it both for the parent language and the child language (using different values). E.g. set core.more=ESTest for 'es' and core.more=ESMXTest for 'es-mx'. For case 2.1, check that the app displays the original string when using the parent language and it displays the custom string when using the child language. For case 2.2, check that the app displays the custom string both when using the parent and the child language. For case 2.3, check that the app displays the custom string when using the parent language but not when using the child language. For case 2.4, check that each language displays its custom string.