The language picker drop-down list is next to the title of the hub page in the Hubs tab, of Hub Designer. Here is an example of the language picker with English (United States) as the default language with Deutsch, English (Ireland) and Espanol as options in the drop-down list for a hub:
The language picker displays the default language for the hub page that you select and any localized pages that were added. In addition, the language that you select in the language picker controls which language is applied to the currently selected hub including the left navigation.