Support @xml:lang everywhere for RDFa compatibility

[jbs1]

migrated from Trac, where originally posted by clange on 8-Jan-2011 10:59am

In addition to CMP and the OMDoc 1.2 metadata fields, @xml:lang should be supported everywhere for RDFa compatibility. The spec should particularly recommend the usage of @xml:lang with the meta element. But technically the attribute should be allowed everywhere, as, technically, RDFa attributes can be present on every element.

[jbs1]

migrated from Trac, where originally posted by clange on 8-Jan-2011 11:20am

for now I have added it to meta (in the 1.3 RNC), as I needed that most urgently.