Move language selection to a dedicated page?

I have been browsing various documentation sites, as part of exploration for #1, and I quite like how React has handled their presentation for the documentation translations: https://reactjs.org/languages

By having a separate page, the page can now contain:

• Clear indicators for the "state" / "completeness" of the translation efforts, like "In Progress" and "Needs Contributors".
• Clear links to contribute to each of the language-specific translation efforts.
• Callout to a documentation page, for folks interested in contributing additional languages.
• A lot more space to present translations across many languages. :)

I think all of these are basically non-feasible for a dropdown-based language selector like we have today on docs.python.org.

Esamina l’attuale selettore della lingua basato su un menu a discesa e confrontalo con la pagina delle lingue di React collegata nell’issue. Definisci gli indicatori dello stato delle traduzioni, i link per contribuire e le indicazioni per aggiungere lingue della pagina dedicata; il lavoro sarà completato quando queste esigenze saranno presentate chiaramente in più lingue.