Is it permissible to intermingle with foreign girls under the pretext of learning a language?

It is not permissible for your friend to associate with a woman who is a stranger to him under the pretext of learning a language. This is a flimsy excuse by which he circumvents the Sharia and his own conscience. Learning a language does not make it permissible to look at women or befriend them. It is incumbent upon you to advise him to fear Allah and beware of associating with strange women. If he does not respond, then avoid his friendship.