Better support for deliberately unused codes/script variants

Some codes are deliberately unused, e.g. https://codelookup.toolforge.org/skr shows everything as untranslated, which is because Translatewiki says "This language code should remain unused. Localise in skr-arab please.", but if you look at https://codelookup.toolforge.org/skr-arab instead, it doesn't have the right project links.