That seems to be an outlier. I'm not a l10n expert at all :smile:  but looking at  http://www.w3.org/International/questions/qa-choosing-language-tags#scriptsubtag the 'Latn' in sr-Latn is a script subtag. Whereas in something like en-US 'US' is a country and should be capitalized.

See also http://tools.ietf.org/html/bcp47#section-2.1.1 Which has this on : 

> [ISO15924] recommends that script codes use lowercase with the initial letter capitalized ('Cyrl' Cyrillic).