bpo-31045: Language switch (#2652)

* Doc: Indicate the language

* Renaming version_switcher to switchers (to add language_switcher).

* Adding language switch.

* Doc switchers: Enhance readability of regex parsing versions.

* Doc switchers: Desambiguate the need of a replace(/\/+$/g, '') by proper naming.

* Doc switchers: py3k can't reach js, it's redirected server-side by nginx.

* Doc switchers: Examples matching actual regexes.

* Doc switchers: Better fallback on unexisting translated version.