Extend I18n setup to allow translated screenshots

[pabzm]

Currently the english docs are produced to /docs/help/1.1/en – the help plugin expects …/en_US though.

The Makefile was changed away from using locales towards using languages in 2015.

There seems to be something missing here, or the change was never pulled through.

Artifacts from previous attempts to internationalize also screenshots were removed, such a setup would be a welcome improvement.

[pabzm]

I fixed the URL fragment to match the published URL, but can't fix the i18n-attempt now.

[pabzm]

I rebuilt the whole setup, including a new more modern theme and text translations for french, german, and slovakian language. Artifacts from previous attempts to internationalize also screenshots were removed, such a setup would be a welcome improvement, and thus I'm renaming this issue to reflect the missing part.