Hi German,
the Mobile app doesn't download the language packs from the server, language packs come inside the binary that is installed on your phone, tablet or desktop (this is why via the app settings you can choose a different language).
In your case, it seems that the problem is that the browser embedded inside Moodle Desktop (Moodle Desktop uses a Chromium browser inside) does not come with support for es_es so it fallbacks to es when it detects that your macOS uses es_es.
Could you upload somewhere a screenshot of the following screen of Moodle Desktop (Configuración -> Acerca de), please, be sure that the section "Agente de usuario del navegador" and "Lenguage del navegador" it is visible in your screenshot.
Juan