Re: “English is the common language of Jews today not Yiddish.” Actually, among frum Jews the common language is Ivrit. And that doesn’t change with definitions of “frum”, i.e. whether one define frum as arguably Shomer Kashrut and Shomer Shabbat , or strictly “hareidi”. While Israelis with university degrees are fluent in Israel, among orthodox Jewish Israelis such degrees are less common, meaning fluency in English is less common.
Yiddish is the native language of a significant part of the hareidi community, but has lost most of its significance otherwise. It should be noted that from an orthodox perspective, Yiddish was never a language of scholarship or serious writing.