15 January 1941,
The Franco authorities forced the football team Barcelona to put its name in Spanish: Barcelona Football Club. A month earlier, the Franco government forced by decree all groups to write their names in Spanish, to remove the words of other languages and English neologisms. This measure also forced Athletic de Bilbao to rename and Atlético de Bilbao until in 1972 they passed the law.
Source:
Vicent Masià, Football Stories: Spanish football in 1940 (La futbolteca, April 2011).