It is fine, but you should write "whose aim" rather than "who aim".Dear Sir/Madam,
I am writing to express my strong dissatisfaction with the Spanish course I attended during the summer holidays.
I stayed with a very unfriendly Spanish family. They were rude to me and were constantly making fun of the Poles.
Moreover, there was supposed to be an intensive grammar course and debating who aim was to improve pronunciation and conversation skills. In fact, the tutor just gave us a handout with grammar exercises and then started reading a magazine instead of explaining the grammar tenses to us! The conversation tutor was not any better. She had told us to introduce ourselves in Spanish and then left the class for 20 minutes. Therefore, the tutors were anything but highly experienced.
What is more, you were supposed to organize a trip to the city museum. I had paid for it and at the very last moment, which is two hours before the trip had been supposed to start, I was informed that the trip had been cancelled! In addition, you did not refund my money.
I feel that I should be fully compensated for my major disappointment. I look forward to a reply at your earliest convenience.