Within the construction of post-war Czechoslovakia as an ethnically, linguistically and culturally homogeneous society, there were bizarre bureaucratic demands placed on the Jewish survivors of the concentration camps. In order to have their Czechoslovak citizenship restored, they had to prove that they were state and nationally reliable. They had to prove officially that they were not German. Nevertheless, they were often still labelled as Germans by officials.
Using German language or having a German accent immediately placed the surviving Brno Jews among the hated occupiers. The anti-German presidential decrees applied to anyone who wrote the word "German" in the "nationality" column of the 1930 census. Such a person had to fight to be granted citizenship and the right to remain in Czechoslovakia, otherwise he or she could be included in the deportation list.
The deportation of Jews as Germans was prevented only by a special decree of the Ministry of the Interior in September 1946, which responded to exaggerated reports in the foreign press about new deportations of Jews from Czechoslovakia.
Why do you think that most Jews in Brno used German in their everyday communication before the WWII?