The meta element supports several attributes which metatag currently ignores, including "scheme", "lang" and "dir".
These attributes are required for the proper output of Dublin Core metadata and other metadata standards, including AGLS.
This patch adds support for these elements using a new class, DrupalExtendedMetaTag, which implements these attributes. It provides an extended form for elements which can benefit from the properties, and translates values and attributes for rendering.
In addition, I've patched the DublinCore metadata set to use the new extended tag.
What this patch probably needs is a update hook in metatag_dc.install to update existing tags, however I'm not sure the best approach for that. If someone can suggest one, I'm happy to take a shot at that.