Consistently support languages for localization

[awvwgk]

Context

This theme supports a subset of languages available with sphinx (https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-language), especially for variants like pt, pt_BR, and pt_PT only a single language pt is supported by the theme.

Proposal

Consistently support all languages available in sphinx, for variants where no specific translation is available fallback gracefully to the parent language.

Tasks and updates

No response