Reported via IRC -- this Esperanto Wikipedia page http://eo.wikipedia.org/wiki/Citrongor%C4%9Da_barbulo was showing a <ref> error message in French.
Running ?action=purge to clear its cache re-rendered the page with the message in Esperanto as expected.
Cite_body::error uses UI language:
wfMsgNoTrans( 'cite_error', wfMsgNoTrans( $key, $param ) ) .
while everything else is content language.
Either this needs to use content language too or it needs to record the use of the UI language in the parser options so it can go into to the parser cache hash key, I think?