How can the slow learning progress of feral children be explained by Descartes? I believe he said that if we don't know something or believe something wrong, it is because we are choosing wrongly (I don't know if this explanation is right). But how would this work in the cases of children who lived for a while in the wild or lived in a very neglected state?
Descartes believed in the theory of innate ideas, the theory that not all of our knowledge is from education and experience and at least some of our knowledge is knowledge that we are born with.
Another classical example of this theory is presented by Plato in the Meno Socratic dialogue. In this dialogue, the slave boy, being young and being a slave, has no education or no scientific or mathematical training whatsoever, so he is similar in essence to your feral children. Yet Socrates, through a series of questions - but without providing any answers - shows that the boy still has a good grasp of geometry. Per Plato, this is proof that the boy was born with this knowledge of geometry already instilled in him.
Where does this knowledge come from? Plato saw it as proof of souls and past-lives. Descartes saw this knowledge as something God put in us. A more modern take might be that this knowledge was programmed into our neural configuration by the process of evolution and natural selection.
Descartes had little interest in cases of abnormal psychology. To take an example, at the outset of his long argument in the Meditations, he declared unhesitatingly that he wasn't mad.
And how could I deny that these hands and this body are mine, were it not perhaps that I compare myself to certain persons, devoid of sense, whose cerebella are so troubled and clouded by the violent vapours of black bile, that they constantly assure us that they think they are kings when they are really quite poor, or that they are clothed in purple when they are really without covering, or who imagine that they have an earthenware head or are nothing but pumpkins or are made of glass. But they are mad, and I should not be any the less insane were I to follow examples so extravagant.
That he wasn't mad was not a conclusion of any argument or proof of Descartes's. It was just a necessary methodological assumption, without which his enterprise would have lost its point. Descartes's meditations were meant for normal and intelligent adults. He had little concern with those outside this sphere. Feral children were therefore largely beyond the scope of his work...