Using Japanese locales on a character-based terminal (TTY) requires that you use terminal settings to make line editing work correctly.
If your terminal is a CDE Terminal emulator (dtterm), use stty(1) with argument -defeucw in any Japanese locale (ja, ja_JP.PCK, or ja_JP.UTF-8). An example in locale ja is:
% setenv LANG ja % stty defeucw
If your terminal is not a CDE Terminal emulator, but the codeset of your terminal is the same as that of the current locale, use stty(1) with argument -defeucw.
If your terminal's codeset doesn't match that of the current locale, use setterm(1) to enable code conversion. For example, if you are in locale ja but your terminal requires PCK (Shift_JIS code), specify:
% setenv LANG ja % setterm -x PCK
See the setterm(3CURSES) man page for details.