Language and country are in the ROMs
A Data East or Sega game shows its country (and so its language) when switched on. The only way to change it is to burn new display ROMs, and the CPU ROM should be replaced at the same time with the US English version.
Systems
Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Three, Clay Harrell (pinrepair.com)