Returns the name of the language given an ISO-639 language code as found in a GST_TAG_LANGUAGE_CODE tag.
The name will be translated according to the current locale (if the library was built against the iso-codes package, otherwise the English name will be returned).
Language codes are case-sensitive and expected to be lower case.
two or three-letter ISO-639 language code
language name in UTF-8 format, or NULL if