Artificial mixture of languages?

Mixing languages does not mean merely using two languages as they actually exist.  It also means creating a mixture that does not exist.  This seems to be the case with a masterpiece of French literature, the Girart de Roussillon (1165-80?), which is “a poem of very high quality, composed in a mixed language exhibiting features of both French and Occitan.”  The poem is “written in an artificial mixture of Provenςal and French.”  Of course, especially with poems in the past, it is very difficult scientifically to determine for sure if a poet is using what William Wordsworth referred to as "the real language of men" (and women!), but for literary criticism, it may not be crucial to the understanding and appreciation of a text whether the poet is merely mirroring reality or constituting it.  What is important is the poet integrates the two languages (whether authentic or manipulated) into a new whole that transcends, while harnessing the resources of, the two.