All MediaWiki version pages (e.g. MediaWiki 1.38) currently show "This page contains release notes for a version of MediaWiki." regardless of their support status.
In fact, on source document pages, {{#invoke:Template translation|getLanguageSubpage}} would always return "en", so the second parameter would always be used, but never the first one, regardless of whether one is on a source document page or a translation subpage.
Please try to fix this problem as soon as possible.
Done by this edit.
I think that {{#invoke:Template translation|getLanguageSubpage}} should return empty string instead of "en".
@Shirayuki, this reverting may be good for fast improvement, but, IMHO, is bad for solution. I have not fully understood the problem yet, but certainly the using of a template {{pagelang}} should be minimized in principle.
Maybe module:Template translation is different from meta and Commons version (i will investigate this), or just my understanding is outdated.
Ok, difference in this edit by @Krinkle. I was wrong. But, in my opinion, this whole structure needs to be simplified and unified between sites - it looks extremely overcomplicated.