According to Wikipedia:

“The experiments were recorded by the monk Salimbene di Adam in his Chronicles, who was generally extremely negative about Fredrick II (portraying his calamities as parallel to the Biblical plagues in The Twelve Calamities of Emperor Frederick II) and wrote that Frederick encouraged ‘foster-mothers and nurses to suckle and bathe and wash the children, but in no ways to prattle or speak with them; for he would have learnt whether they would speak the Hebrew language (which he took to have been the first), or Greek, or Latin, or Arabic, or perchance the tongue of their parents of whom they had been born. But he laboured in vain, for the children could not live without clappings of the hands, and gestures, and gladness of countenance, and blandishments.’”