The twoLetterCode type defines a string of 2 characters, only (capital or small) letters.
Example:
<languageCode>EN</languageCode>
XML Schema:
<s:simpleType name="twoLetterCode">
<s:restriction base="s:string">
<s:pattern value="^[A-Za-z]{2}$"/>
</s:restriction>
</s:simpleType>