A large percentage of Klal Yisroel, Bnei Torah, Orthodox Jews, speak Yiddish as their first language. Even if English is their second language since they live in the United States, Canada, the United Kingdom, Australia, etc.
Certainly and indesputatably such a large percentage of Frum Yidden need to be accommodated by the Siyum HaShas. Especially given that they are a huge percentage of those who learn a daf a day and complete Shas on the seven year cycle. And that a large percentage of the Daf Yomi shiurim are given in Yiddish.
Not to even mention that the Daf Yomi was founded in Yiddish by Yiddish-speaking rabbonim.