Region encoding is the mechanism that enables motion picture studios to control the worldwide release of their movies. It is required by the DVD Forum (http://www.dvdforum.org/forum.shtml) in all commercial hardware DVD players. Every DVD-Video disc contains one byte of data representing a region code, which limits where the disc can be played.
Once you have set the region in DVD Player five times it cannot be changed.
Instead, use VLC to view videos from different regions: