As it says (https://gerrit.wikimedia.org/r/gitweb?p=mediawiki/core.git;a=blob;f=resources/mediawiki/mediawiki.jqueryMsg.js;h=6e2d3b410b33cfb8f2835ee6f1457b7dc264f35f;hb=refs/heads/master#l156) this functionality should be moved to the server.
There is no logical reason to put it on the client. The current structure is confusing. Among other things, the docs say to use mediawiki.jqueryMsg only when you need PLURAL/GRAMMAR/GENDER. But it's actually needed for any message with e.g. an external link (it also has part of the infrastructure for wikilink parsing, but not there's a missing piece, wlink; see https://bugzilla.wikimedia.org/show_bug.cgi?id=43498).
Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=25349
https://bugzilla.wikimedia.org/show_bug.cgi?id=43513