[web-annotation] Issue: Relationship between dc:language and processing language for multilingual resources marked as i18n-review

r12a has just labeled an issue for 
https://github.com/w3c/web-annotation as "i18n-review":

== Relationship between dc:language and processing language for 
multilingual resources ==
For multilingual resources (i.e. dc:language) has multiple values, is 
it mandatory that the value of the processingLanguage must be one of 
the values in the dc:language?

If this supposition is true, please add a non normative note that the 
"Processing language should have one of the values available in 
dc:language" 

See https://github.com/w3c/web-annotation/issues/343