About [[MediaWiki:Expensive-parserfunction-category-desc/en]]

Just add the URL to the translation. There's no need to add this as a parameter.

In general if there is nothing to localise in a longish url then why not hide it behind a parameter (as done in MediaWiki:Newarticletext/en)? I'd say it makes it easier to distinguish what needs translating. The message in question is actually a good example of this.

First now when I looked at the url more closely did I realise that only two thirds of what I saw was the actual url. It actually says [https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:$wgExpensiveParserFunctionLimit Manual:$wgExpensiveParserFunctionLimit] (my emphasis)

I.e. Manual:$wgExpensiveParserFunctionLimit is coded as the link text. Since our general recommendation is to not mess with anything looking like variable names this will leave an English link text in most languages.

I believe that changing it to [$url the manual] or even [https://... the manual] would help the translatability of the message.