My RSS feed suddenly stopped validating – with:
This feed does not validate.
line 16, column 11: language must be an ISO-639 language code: [help]
Looking at the PHP file I see that the language is worked out thus:
<language><?php echo get_option('rss_language'); ?></language>
if you change it to :
<language><?php echo bloginfo_rss( 'language' ); ?></language>
it starts validating again.