Shinjiman and Happy Rabbit, I don't know about everybody else here, but I'd find this issue easier to think about if I understood a bit more about what you're trying to accomplish. Apologies for not knowing as much as I should about these issues.
My main question is this. Why do we need the extra mapping of the HTML lang tag? Why is it not adequate to set the value of $wgContLanguageCode appropriately for each wiki, so that it matches the language that the wiki's pages are written in?
(I'm ignoring the question of interwiki links, and of the wiki's customized pages, because I think those are separate issues, and not the one I think we're most interested in here.)
I can guess what the problem might be, but I'm not sure. Please tell me which of the following scenarios it is, or if it isn't any of these, please explain what the issue really is.
1. Perhaps, within one wiki, there are different pages written in different variants of the language. Since $wgContLanguageCode is constant across the whole wiki, it can't be correct for each page. It must be overridden depending on the page being displayed.
2. Perhaps some browsers are broken, and don't choose the correct font based on the HTML lang tag. So even if $wgContLanguageCode is set correctly for the language a wiki's pages are written in, some users will see it incorrectly. So those users need a way to override the value of $wgContLanguageCode.
3. Perhaps the problem is this "Language Converter" thing. Even though a wiki's pages are written in one language, that language can be automatically altered, on the fly, as the page is displayed, to some other variant of the language. But when this is done, the value of $wgContLanguageCode for that wiki is no longer correct for the language being displayed.
Also, i don't understand what information would be used to adjust the HTML lang tag. Brion has suggested that $wgContLanguageCode should always be used, that is, that
html_lang_tag = $wgContLanguageCode
You're suggesting that the HTML lang tag has to be derived as a function of $wgContLanguageCode and some other information:
html_lang_tag = f( $wgContLanguageCode, ??? )
The question is, what is the other information? A per-user preference? Extra tags on the page being displayed?