Fix EZP-19856: [HTTP].ContentLanguage settings in locale files updated; ...

...closes #671

I tried to to simplify ContentLanguage codes using IANA (http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry) document and W3C recommendations (http://www.w3.org/International/questions/qa-choosing-language-tags). Not all have been shortened to 2 characters.

For the possible discussion: I had doubts about (* means chosen):
*`zh-CN` or `zh` or `cmn` or `zh-Hans` (China)
*`es-ES` or `es-ES` (Spain)
*`es-MX` or `es-419` (Mexico)
*`fr` or `fr-BE` (wikipedia http://en.wikipedia.org/wiki/Belgian_French sais "Belgian French and the French of northern France are almost identical, but there are a few distinct phonological and lexical differences.") (Belgium)
*`fr-CA` or `fr` (Canada)
*`pt-MZ` or `pt` (Mozambique)
*`sr-Latn` and *`sr-Cyrl` or `sr` (Serbia)