Maintenance: Add language switcher to the page header.

[dvuckovic]

@ralf401 @mgruner I think we should apply this only to the main branch, since we don't publish translations for the pre-release. What do you think?

[mgruner]

I think we can't prevent it from going to pre-release as well (latest with the next merge/reset). It must be robust enough to detect that. How does it work for the pre-release warning badge that is there currently? It must detect this somehow, based on build environment data.

[mgruner]

OTOH, pre-release can also be translated. We just don't publish the translations (yet).

[dvuckovic]

I think we can't prevent it from going to pre-release as well (latest with the next merge/reset). It must be robust enough to detect that. How does it work for the pre-release warning badge that is there currently? It must detect this somehow, based on build environment data.

Unless we don't reset it anymore? Note that pre-release is also the default branch of the repo: all clones have to reset their state which is really an inconvenience.

Warning badge is powered by the versionwarning extension for Sphinx. So, it's injected at build time, while our new language switcher is currently run time only and is always injected.

I found a way to skip the injection of the language switcher in pre-release mode. However, I did build in a condition to show the language switcher only if translations are enabled: it should work as long as the number of link sections in the RTD panel does not change (a bit flimsy, but works well). This will be needed for future stable versions, since they will have translations in place.

[mgruner]

Looks very nice!

[ralf401]

Looks really nice!!