I would have said "written in dialect." I too would be interested in a more succinct term. The word "dialectically" might fit. The poetry of Robbie Burns is usually said to be "written in dialect" and I have started posting entries from a 100 year old "Blegburn Dickshonary" written in Lancashire dialect (see http://0042bfe.netsolhost.com/dickshonary I’m looking for volunteer proof-readers)