Jothar: in fact, I have learned large portions of the sefer.
There is no Halacha with regard to Yiddish over any other language, i.e., it’s assur to talk during davining whether in English, Yiddish or Lashon Hakodesh.
As gavra_at_work pointed out earlier, the gemorra considers ancient Greek to be on a higher level of kedusha than other goyish languages. Yiddish of course had not yet evolved in Talmudic times.
So while there is no language on the holy level of Loshon Hakodesh or even ancient Greek, Yiddish is considered “holy” in the abstract for all the reasons shown previously.