Is it permissible to befriend an eighteen-year-old Swedish girl solely for the purpose of language learning?

It is not permissible to associate with a foreign woman, whether for learning a language or for any other purpose, especially if she is a disbeliever. Allah has forbidden befriending or showing affection to disbelievers, as He says: "O you who have believed, take not the Jews and the Christians as allies. Some of them are allies of one another. And whoever of you takes them as allies - indeed, he is of them. Indeed, Allah guides not the wrongdoing people." (Quran 5:51) And He says: "You will not find a people who believe in Allah and the Last Day loving those who oppose Allah and His Messenger, even if they were their fathers or their sons or their brothers or their kindred." (Quran 58:22) This is even more reprehensible if the woman comes from societies where nudity and moral decadence are prevalent. One must fear Allah and seek other ways to learn the language.