Category:Intro/*
They are created when pages are listed in them. There's a tracking category for that. The verbal request with instructions is in the main category of the site since its origin in February 2009 (even before you joined this site) by Siebrand, with a small modification by Kaganer a short time after to make this even more explicit. So this is not new and its originating from the origin of this site!
Note: I wrote "on demand", not "on request" (there's no new request posted in some talk page, the demand comes from usage on translated pages). There's not been any change of rules.
During many years, managing categories was left administered, and they started proliferating and being unorganized or named inconsitantly. Slowly they were reintegrated. Now the iste only has 1 uncategorized category, the main cateogy in English. All pages are navigatable by categories. And when you say they are not referenced, you're wrong. All pages have at least one category. Their number does not explode, but they are all organized by language as instructed, Categories don't need to have lot of members, just to be consistant. They'll populate slowly as soon as some translated page needs one of them.
Can you please give a direct link to the page where Siebrand and Kaganer wrote it?
You wrote specifically: "Yes they were essentialyl created by me, but because they were requested" (sic). I'm trying to understand that request.
Oh, do you mean the text of Category:translatewiki.net itself?
I interpret it as an instruction to use the [[:Category:{{SITENAME}}]] and [[Category:{{SITENAME}}{{#translation:}}]] syntax when it's referenced, and I'm not even sure that this complex syntax is very useful for anything. There's nothing in those instructions that says to manually and intentionally create "Intro/*" categories for every language. That's neither a request nor a demand.
It's also quite confusing that Category:Intro/ab includes the page Translating:Intro/ab, but when you look at Translating:Intro/ab itself, it is categorized under Category:Intro. It's the same for Translating:Intro/he and Translating:Intro/cy.
Translating:Intro/fr points to Category:Intro/fr, which is actually useful because it includes multiple pages. But for most other languages, it just serves no purpose.
You looked badly. Translating:Intro/ab is in Category:Intro/ab, and so on for Translating/he or Translating/cy.
Translating:Intro/he is under Category:Intro right now. Translating:Intro/ab was under Category:Intro when I looked at it yesterday, and now it's indeed under Category:Intro/ab; I guess that something was updated in some caches. But OK, now it's under Category:Intro/ab, and what now? What is this one-page category for?
You still haven't answered the main questions: Who asked for these categories? Who thinks that they are useful except yourself? What are they good for except raising your edit count? You say that it's for "navigation", but navigation from where? Most of these pages have just one page, and some of them have zero pages, like Category:Intro/dv. There are no links to these categories from anywhere.
A lot of the Translating:Intro/* pages are barely actually translated; many of them, like Translating:Intro/ta, only have the title translated. Just because of that title and because one of the templates has [[Category:Intro{{#translation}}]], a red link to Category:Intro/ta is created. Because of how categories work now, to make it blue, the category has to be manually created. But is it actually useful to make it blue?