In T201613#9421084, @Winston_Sung wrote:

T353870: editintro no longer does language conversion since MediaWiki 1.41+ might be caused by https://gerrit.wikimedia.org/r/910112/ ( 6a20dc2 , If0b05710cb52a977bf4e85947d72d68683a0a29e ).

Good guess, that looks like the cause.

The old code parsed the custom edit intro in "content" mode:

				$this->context->getOutput()->addWikiTextAsContent(
					'<div class="mw-editintro">{{:' . $title->getFullText() . '}}</div>',
					/*linestart*/true,
					$this->mTitle
				);

The new one uses the default mode for localisation messages, which is the "interface" mode:

					$localizer->msg( new RawMessage(
						// Added using template syntax, to take <noinclude>'s into account.
						'<div class="mw-editintro">{{:' . $introTitle->getFullText() . '}}</div>'
					) )->parse()

The reason for this was not documented, so I thought it was a mistake (as it was definitely a mistake in many other cases I've seen that method used).

Change 984842 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):

[mediawiki/core@master] Parse custom edit intro as content to enable language conversion

https://gerrit.wikimedia.org/r/984842

Change 984842 merged by jenkins-bot:

[mediawiki/core@master] Parse custom edit intro as content to enable language conversion

https://gerrit.wikimedia.org/r/984842

Change 989928 had a related patch set uploaded (by Winston Sung; author: Bartosz Dziewoński):

[mediawiki/core@REL1_41] Parse custom edit intro as content to enable language conversion

https://gerrit.wikimedia.org/r/989928

Change 989928 merged by jenkins-bot:

[mediawiki/core@REL1_41] Parse custom edit intro as content to enable language conversion

https://gerrit.wikimedia.org/r/989928