This is the line that needs to be changed:
Set up your developer account with gerrit following the directions here: https://www.mediawiki.org/wiki/Gerrit/Tutorial
Then clone the TemplateData extension (https://gerrit.wikimedia.org/r/admin/repos/mediawiki/extensions/TemplateData), make changes to the en.json file, and submit those changes for review to gerrit following the directions in the gerrit tutorial.
The TemplateData parameter type "wiki-page-name" is labelled just "Page" in English (https://translatewiki.net/wiki/MediaWiki:Templatedata-doc-param-type-wiki-page-name/en). There is no doubt what the literal translation of "Page" is, but the problem is that "Page" as well as its literal translation is ambiguous if you don't already know what it refers to in this case: Does it mean "Page number" or "Page name"? As soon as you know it means "Page name" there is no problem, but not everyone knows that or even remembers it, and I believe we should make everything as self-explanatory as possible. It just makes things easier and avoids misunderstandings.
The TemplateData parameter type is also not called "wiki-page" but "wiki-page-name".
So I suggest we change the English (original) label/string from "Page" to "Page name".