Resolve the feed filename for a language. English publishes to the canonical rss.xml; every other language gets rss_<lang>.xml.
rss.xml
rss_<lang>.xml
Language code
Feed filename (e.g. rss.xml, rss_sv.xml)
rss_sv.xml
Resolve the feed filename for a language. English publishes to the canonical
rss.xml; every other language getsrss_<lang>.xml.