Most of the content retrieved by the app is filtered at server side, before the app receives the content. That means the app will only receive the text in the "final" language, not all the languages. By default this filtering will use the language selected by the user in LMS (the selector you sent), but the Moodle app sends Moodle a parameter to indicate which language to use instead of the one selected by the user. The language sent by the app is the one selected in App settings.