My guess is it requires a parts change from the dealer, but it could be on the MFD display screen as shown in the attached excerpt from the book in the glove box. The pages I have included are from a 2008 which may or may not be the same as the 2006 and don't show Japanese as a language option. Both are the same generation. I would think Japanese was not sold in the US but since you did not put your location in your profile or the question, it's only a guess.
OP is probably from NZ/Australia or UK since those are among the countries with RHD vehicles and Japan domestic market used vehicles are sometimes imported into those countries. As previously stated, it is probably required to replace the MFD at minimum. There would be low demand in the JDM to select any languages besides Japanese.
