Hebrew vs Yiddish
Maybe an Israeli can confirm the accuracy of this- In my experience in the USA the heredi community still speaks Yiddish or other European dialects at home and with others in the community.
Perhaps they used Hebrew in the film because virtually no one else but heredi and other Jews in their 80s and 90s speak Yiddish any more? For the film to have a more widespread language appeal in other words? At least in Israel, where almost all citizens speak/understand modern Hebrew.
I'm fairly sure most heredi Jews don't like modern Hebrew and believe the Hebrew language should be confined to studies and spoken for religious purposes only. In other words- It is a holy language not suited for casual use and outside of religious traditions.