Any GBA game from any region will work with any GBA system. But like someone said, the same games from different regions might not work together in multiplayer (some do, but most don't I think).
The battery charger from different regions accepts different levels of AC voltage though, so you'll need a converter to use it in a different region (USA and Japan can be used interchangably I think, since the voltage difference is very small), or you can buy a replacement charger from your region so you can just jack it into a wall socket without any power adapters.