acpkendo:
I think you mean the SL-B500; there was no SL-B400. The SL-B500 is the Japanese version of the SL-5600, not the SL-5500. Therefore, I wouldn't suggest trying any of the B500 roms (specifically kernels) on a 5500.
As for Japanese input, have you tried installing the Japanese support ipk that is floating around? It was built using files taken from the C-series devices, so I have no idea if they'd work or not on a 5500, but it's worth a shot. If you try this, please let me know the result.
- ashikase
anpachi, gifu, japan