I asked, a bit ago when setting up some private wikis using the new features in 1.17, how one finds out which messages are modifiable, and where to find them to change them. Specifically, I was trying to locate the messages on the login page you're presented with when arriving, un-logged-in, at such a wiki.

I think that this:

"Another help tool is the qqx language code: there is no associated MessageQqx.php file, and it isn't possible to select this fake language in the user's preferences. But when used with the &uselang parameter to display a wiki page (e.g. https://en.wikipedia.org/wiki/Special:RecentChanges?uselang=qqx), MediaWiki will display the message keys instead of their values in the user interface: this is very useful to identify which message to translate or change."

from Guillaume's technical writeup at:

https://www.mediawiki.org/wiki/MediaWiki_architecture_document/text

might be the answer I was looking for. I document it here for the archives.

Cheers, -- jra