Polite Japanese

Yesterday, I made friends with David, who entered ICU in September. He has been learning Japanese since he was in America. When we introduced each other, his Japanese surprised me very much. He said "David to moushimasu. Yoroshiku onegai itashimasu." Also, when my friends, Mio, Kanako and I asked some questions, he answered "Hai" or "Iie"! Do we use such a polite phrase? I suppose many people answer "Un" instead of "Hai". I was shocked by this incident.
Recently, a lot of young people have been using bad, incorrect Japanese. There are many words called "Wakamono-kotoba". I used them a lot when I was in high school, and I thought it cool. Words or phrases I used made sense only among my friends, so my mother always said, "Please stop using such a bad Japanese! I can't understand anything!" Now I know her feeling. I never hear the conversasion of young girls without feeling uncomfortable.
Many people say that Japanese is one of the most beautiful languages. I strongly agree to that. Japanese has a lot of wonderful words related to nature, seasons, and also has "Keigo", which expresses respect for others. They sounds a little old-fashioned especially for young people, but I think we should be proud of our language and try to speak POLITE Japanese. Before we learn English or other languages, we ought to think about our own language again.