I'm not entirely sure, but it might depend on the order the language packs were enabled in. I don't think there's any code in Flarum that specifies a sorting, but maybe the Symfony translator that we use behind the scenes sorts the locales in a predictable way.
If you want to force a specific order, you could use a PHP content extender to modify the app.data.locales
object which is used by the frontend to build the dropdown.