I am a big advocate of early language learning - we can pick up so many things later in life, but languages are not generally one of them (to native level, at least!) I really wanted to cover as many languages as possible with my son, but just couldn't manage consistancy with so many and pared it down to just four, two native and two foriegn. For now
I agree that it is possible to learn languages through dvd - I have no Mandarin or Japanese myself, but since they are the most linguistically different to English of the languages I wanted to cover I have focused on them first of all. My son now insists on naming dogs only in Chinese (I think this is due to Little Pim's dog films) yet has only heard the word on dvd. He refuses to use English or Russian words for dog, though he does understand them.
It is undoubtedly true that babies can't get a native-level understanding of a language through dvd alone, but it
is an important resource for those without access to native teachers to at least introduce the sounds and workings of a language.