Commercial Dreamcast games will only work with Dreamcasts of those regions that they've been published in, while homebrew, independent games (Feet of Fury, Last Hope, bleem!cast, etc.) and backups do work on all Dreamcasts.
If you really want to avoid using a boot disk, you could set your DC to Japanese with DC-Swat's Region Changer:
http://dcemulation.com/phpBB/viewtopic.php?f=31&t=89863
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