Most Japanese DVDs do have Japanese language dubs as well as original soundtrack... in the case of the Disney DVDs that the original poster is asking after, the Lion King, Little Mermaid, and Aladdin are all discontinued. Anastasia
can be found at Amazon Japan for 2,500 yen and has Japanese language track as well as Japanese subtitles, Mulan is no longer available but there is a second hand copy available from their Market Place (for over 9,000 yen!!). I could not find Hercules at all.
Hope this helps.