Wien Cod. 13647

[eggyk]

Great observation! I totally missed the "Aberillen"! What language would this be? It is certainly not German, maybe some weird Occitan dialect?

It is certainly a dialect of german. Bavarian-austrian. 

mertzen, abrillen, may, ander may, erst august, ander august, erst heribst, ander heribst, erst windter, lest windter

The sequence of first may, other may, first august, second august, first autumn, second autumn was common in german speaking areas. Heribst is a rare spelling, but was seen in a Salzburg sequence from the early 15th century as well. 

lest (or letzst) in december is a common southern german/austrian thing as well.