Lots of pages on mediawiki.org have "<translate>" and "<!--T:123-->" bits strewn about, making them hard to read and edit in source, and hard to edit in Visual Editor.
Translation should not rely on making large parts of pages uneditable!
Per discussion it looks like some small tweaks to VE's treatment of the extension tag may simplify things without introducing too much breakage, until a more VE-native solution is available.
To-do: add detail bugs and replace this one.
See also: use cases for which <translate> tags are considered better than structured translation: T116235: [Epic] CentralNotice translation should move closer to MediaWiki i18n standards and the code cleaned up