All MessagesXX.php files use double-quoted strings in the $dateFormats array to include characters that would otherwise be interpreted as date formatting placeholders.
Only files that deviate from this pattern, instead using backslash-escaped characters:
MessagesGl.php, MessagesKea.php and MessagesPt.php.
They should be normalized to the same syntax used in the other files.
(Note: I'm creating this task retroactively, to accompany the Gerrit patch 804770.)