With JS disabled, the message shown is parsed, but is "Folgende Vorlagen werden von diesem Artikel verwendet:" — which doesn't seem to be either the one given above, or the default for templatesusedpreview. (Or am I just looking at it wrongly?)

Anyway, it looks like the issue is with JS parsing the message. This works:

{{PLURAL:$1|Template|Templates}}

and this doesn't:

{{PLURAL:{{{1|$1}}}|Template|Templates}}

Why is there a template variable there? Is that so this message can be transcluded in the wiki?

In T325327#8472894, @Samwilson wrote:

With JS disabled, the message shown is parsed, but is "Folgende Vorlagen werden von diesem Artikel verwendet:" — which doesn't seem to be either the one given above, or the default for templatesusedpreview. (Or am I just looking at it wrongly?)

Anyway, it looks like the issue is with JS parsing the message. This works:

{{PLURAL:$1|Template|Templates}}

and this doesn't:

{{PLURAL:{{{1|$1}}}|Template|Templates}}

Why is there a template variable there? Is that so this message can be transcluded in the wiki?

I think for the German language variants de-at, de-ch, and de-informal.

Before writing this task I tried a more simple version

{{PLURAL:$1|Die folgende [[Hilfe:Vorlagen|Vorlage]] wird|Folgende [[Hilfe:Vorlagen|Vorlagen]] werden}} von dieser {{#ifeq:{{NAMESPACE}}|{{ns:0}}|Artikelvorschau|Seitenvorschau}} verwendet:

without success. Still not parsing.

This is a known limitation: https://www.mediawiki.org/wiki/Manual:Messages_API#Feature_support_in_JavaScript

{{PLURAL:}} is supported by mediawiki.jqueryMsg, but not {{#ifeq:}}, remove that part and plural works (at least locally).

Not sure if the message is returned as plain, when one parser function cannot be expanded, but it looks like that.

I edited the local messages; don't know if jqueryMsg getting enhanced is likely, if not the task could be closed.