Isn't the idea of Sudetenland more or less a 20th century idea? I believe that in the 19th century people wouldn't have realy cared that much about what language the inhabitants of a region spoke.
They were starting to.
Iirc Wilhelm I wanted to annex Saxony and also Austrian territory corresponding to most of what would later be called the "Sudetenland". Bismarck, for good or ill, argued him out of it in favour of other annexartions. The King's proposals make little geographic or economic sense, so I can only assume that he had nationality in mind. He wanted to take some Germans away from Austria, but not too many Czechs.
As for the term itself, I'm not sure when it was coined - possibly only in the 1930s as a convenient "shorthand" for the areas Hitler was claiming. In the WW1 period I think it was sometimes called "German Bohemia", though that too is misleading as much of it was in Moravia and Austrian Silesia.