$wgHooks['OAuthReplaceMessage'][] = function ( &$msgKey ) { if ( $msgKey === 'mwoauth-form-privacypolicy-link' ) { $msgKey = 'wikimedia-oauth-privacy-link'; } return true; };
We can use WikimediaMessagesHooks::onMessageCacheGet instead...
The messages might want moving which file they're in (into wikimediaoverrides)