Sometime prior to 1920, the Slovenian population of Bethlehem, Pennsylvania, stopped being Slovenian. Stephen C. Antalics explains why:
A review of the archives of Ellis Island ship manifests for these first immigrants reflects only Slovenian ethnicity. The 1920 federal census for these same immigrants in Bethlehem reflected them as 100 percent Windish. So, were they Windish or Slovenian? And why is Bethlehem the only place in the world where the term "Windish" is used today?
The Hapsburg Empire had two Slovenian communities, one in Austria, the other in Hungary. This gave rise to severe dialect differences. Dialogue was very difficult. The Hungarian government dispatched clergy to America to encourage its Slovene immigrants to preserve Hungarian sympathies over their Slavic ethnicity by becoming "Wends." This plot failed in every U.S. community -- except Bethlehem. [Link]