The German language in the novel Peace Shall Destroy Many represents a refusal to change and adapt and an adherence to traditions that have long since past. In the 1940s in Saskatchewan, the Mennonite community has mandated that the members only speak in German, adhering to principles that Mennonites practiced when they originated in Germany.
This is indicative of the church at large refusing to adapt and incorporate itself with the rest of the world. They take literally the idea "Be in the world, not of the world" and refuse to associate themselves with anything in the North American continent, because they are following Mennonite tenants.
The church members similarly only accept the teaching and dogma of their Deacon. This causes them to reject some interpretations of scripture that would encourage unity, which they desperately need. Their refusal to change any antiquated customs hinders their missionary efforts and keeps them secluded and isolated, much like one would in modern days if they were living in North America and spoke only German, one of the least common Western languages in this hemisphere.