Custom class for supporting XSD data type language
language represents natural language identifiers as defined by [RFC 1766].
The value space of language is the set of all strings that are valid language identifiers
as defined in the language identification section of [XML 1.0 (Second Edition)].
The lexical space of language is the set of all strings that are valid language identifiers
as defined in the language identification section of [XML 1.0 (Second Edition)].
The base type of language is token.