Steps to reproduce:

(from comment #0)

page[,] move [it] to a different namespace

Hmm.. this does not only happen after page moves. reported at Wikimedia Forum today

By the way, this thing that I posted at Wikimedia Forum happens to me only on Meta-Wiki. On MediaWiki while editing the help namespace in / page display title / the problem didn't show. Don't know if this information would be useful. I just feel like I have to mention this.

UPD: This "changed namespace" message didn't show when I edited / page display title / here, though it showed up while I tried to translate Help: to Довідка: here

Actually, I'd rather see this check from line 299 of MediaWikiMessageChecker.php reworked to omit the Help namespace or at least give users the option to choose which namespaces to filter against with a global array of namespaces to check against.

$namespaces = 'help|project|\{\{ns:project}}|mediawiki';

Perhaps I am missing the point, but why would there be a restriction and then obsolete a title unit just because it has Help as the namespace missing in the title translation. It would make a little more sense (a little) if the namespace itself was a translatable unit.

Right now I am faced with the decision of having to hack this extension to remove that restriction or rename the Help namespace to something else.

reworked to omit the Help namespace

Seems sensible. In fact, it's possible that only "MediaWiki" has a compelling reason to be there (ns8 is definitely special, editprotected and can be used for all sorts of abuse).