Add instance schematron rule: Explicitly name the languages used (BP CS3)

[twagoo]

Best practice CS3: Explicitly name the languages used

The default language for element contents is English (see CS7). Additionally, other languages may be used, complementing the English version of a CMDI record. The possibility of multilinguality has to be already included in the design of the component. For the specification of the language applied, each element should be provided with an @xml:lang attribute (even when the content is in English). Use a BCP 47 language tag to unambiguously identify the respective language within @xml:lang.

For elements that use vocabulary entries translations in other languages then English can be indirectly specified by language specific labels in the vocabulary