Make navbar link to translated version if it exists

[mxmehl]

Currently, all links in the navbar link to the English version, even if viewing a translation and if there was a translation for that page the link leads to.

Unfortunately, the customised relref shortcode does not work here as it would have to be a partial, and I cannot get it to work.

Alternatively, we could just link to it and depend on the .htaccess rule to link back to the EN page if it's not present. It would work immediately in the deployed and Docker development version, but in local previews (with hugo server) it would not work.