Joseph,
I was pointing out that your 2 statements (of Gedolim and 1000 yeats) contradict each other.
But even in the last 200 years, why was Yiddish used? I would say for convenience; that’s what they all spoke. Not because of Kiddushah.
And if you say Yiddish has Kidushah, at what point did this Kidushah come upon it?