When a message fetched with wfMsg() is not defined *for the user's
selected language* in either the MediaWiki: namespace (with language
suffix) or the language file, the language class will fall back to the English-
language default in Language.php.
An example where this is problematic is customized menu items defined in
$wgNavigationLinks; since they are not present in Language.php, an
unsightly <message> is displayed.
It may be better behavior to fall back to the MediaWiki: copy. Probably the
getMessage method should return false, and wfMsgReal can handle the
T10960: [Fallback language] MediaWiki:Key/en messages are ignored when en is the wiki default language
T50956: Can't override optional message in all languages with local customisation
T13267: User should be able to set fallback language(s) in preferences
T40662: Document how fallback language and message loading order in general works
T57473: i18n fallback uses message from i18n file before customized message from DB (MediaWiki: namespace)