I'd assume it's about the dialects, which are very different from the "official" language, so much that Jerome K. Jerome had a joke in one of his books, more than century ago:

"Germany being separated so many centuries into a dozen principalities, is unfortunate in possessing a variety of dialects. Germans from Posen wishful to converse with men of Wurtemburg, have to talk as often as not in French or English; and young ladies who have received an expensive education in Westphalia surprise and disappoint their parents by being unable to understand a word said to them in Mechlenberg."

"Modern times" and technologies suppress the dialects and with each generation the portion of the local-specific dialects is being lost.