You do not need to use "to". If one were to like German more than French, they could say, "I prefer French less than German." Swap "less" with "more", and the sentence is still correct with the opposite meaning.
'Er mag lieber Französisch als Deutsch?' When I translate this, those two words are swapped. Can some explain what's going on here? Are both acceptable? Why?
'Er mag Französisch mehr als Deutsch' means "He likes French more than German"