Originally Posted by ixtab
Ooops. No, it's not critical at all, it's just a lapsus (I forgot to treat en_US as a special case).
You might want to treat en_* as a special case, as I get a similar warning here in en_GB, and I suspect en_AU and en_CA would see something similar.
(In general, locale foo is supposed to be used instead of foo_BLAH if foo_BLAH is active but no specific localization data is available for that locale.)