I am unable to assign the "authority" attribute to the element using the workbench. As a result the METS file generated by the workbench is invalid.
I still need to test this fix completely, but I think the problem is solved. The crosswalk editor now allows you to add the authority attribute to languageTerm (and similar situations), then you can pick a default value in the properties view. In the case of authority and other attributes that are constrained in the same way, the default value is chosen from a list of valid terms in a drop-down menu.
I'd still like to implement something to add required attributes when elements are created, but that will take more time.