lista = [['DE','DO','OU'],['AE','YHH','OO'],['OW','LA','FOR']]
lista_lower = [[w.lower() for w in line] for line in lista]
print(lista_lower)
bGlzdGEgPSBbWydERScsJ0RPJywnT1UnXSxbJ0FFJywnWUhIJywnT08nXSxbJ09XJywnTEEnLCdGT1InXV0KCmxpc3RhX2xvd2VyID0gW1t3Lmxvd2VyKCkgZm9yIHcgaW4gbGluZV0gZm9yIGxpbmUgaW4gbGlzdGFdCnByaW50KGxpc3RhX2xvd2VyKQ==
[['de', 'do', 'ou'], ['ae', 'yhh', 'oo'], ['ow', 'la', 'for']]