Thirty-odd (very odd) years ago when I was at University, my Old Norse professor introduced us to the dialect maps of the Germanic languages on the European continent produced by the Brothers Grimm (of fairy tale fame). What was fascinating was the way different aspects of the language changed at different places. So the border between one pronunciation of a particular vowel, consonant or word and another pronuciation of the same vowel, consonant or wrd did not always correlate with the borders between another pronunciation or word choice shift. So while politicians and educators and other folks with various ethnic or nationalist agendas could try to define the "correct" version of their language, the situtation on the ground was much more fluid. And especially in those days (19th century), when transportation was much more limited. You really got the feeling that if you moved from one place to another slowly enough, spending a few weeks or months at each closely placed stop, you could travel all over Europe -- or at least the Germanic portions thereof -- without conciously learning a new language. And the same principal could have applied within the Romance regions or the Slavic regions as well.