I'd like to buy some movie DVDs in Hong Kong, Taiwan and Japan. People here in the US are telling me to be careful because DVDs bought in those countries might not be formatted correctly for DVDs players in the USA making it impossible to view them in the US, Is this true?

I have also been told by some people that I can buy DVDs that are formatted to play in Cantonese, Mandarin with or without English subtitles? Is this true or is it only special DVDs that can somehow "switch languages"?