Modify the "warning" message when looking up for languages not supported
Yep the latter (unclosed tag) is already in bugzilla. So is just rewording the message enough? Without the help link?
That's also the most difficult part because it would be twn specific, and we can't do local overrides here easily.
Well, if we can't do overrides here easily, then the link is equally valid for all MediaWiki instances; if we enable languages here with methods not valid for every MediaWiki, then the help page may not help.
How about writing a general help page on languages at Mediawiki.org, and pointing to that. That page could contain a link to the languages page at translatewiki.net.
Could be a section in mw:Language in MediaWiki, or Translating:MediaWiki.
I have drafted a new section to be part of the mw:Manual:Language page. I have put it on the talk page for comment. I would be grateful if you would comment there or here before I shift it to the page itself. Have I written anything incorrect or misleading? Have I missed out anything important?
The section is on a page which seems to be primarily aimed at developers, so it looks a bit out of place, being primarily aimed at potential users and translators. However, it seemed to be too specific for the more general mw:Manual:MediaWiki_architecture#Languages page.
About the best place to put the advice needed, if the error message above is part of the translate extension, then I think it would be better if the error message linked to one of the help pages for the translate extension at Mediawiki. The help provided wouldn't then need to be as Mediawiki specific as the help page already drafted for the mw:Manual:Language page.