Czech language
The Czech dialects spoken in Moravia and Silesia are known as Moravian (moravština). In the Austro-Hungarian Empire, "Bohemian-Moravian-Slovak" was a language citizens could register as speaking (with German, Polish and several others). Of the Czech dialects, only Moravian is distinguished in nationwide surveys by the Czech Statistical Office. As of 2011, 62,908 Czech citizens spoke Moravian as their first language and 45,561 were diglossal (speaking Moravian and standard Czech as first languages).

Q: Where could citizens register "Bohemian-Moravian-Slovak' as being their spoken language?
the Austro-Hungarian Empire