updated Japanese translation

For an unknown reason the Japanese.nsh file of NSIS 2.5 misses the entries

!ifdef MULTIUSER_INSTALLMODEPAGE
${LangFileString} MULTIUSER_TEXT_INSTALLMODE_TITLE
${LangFileString} MULTIUSER_TEXT_INSTALLMODE_SUBTITLE
${LangFileString} MULTIUSER_INNERTEXT_INSTALLMODE_TOP
${LangFileString} MULTIUSER_INNERTEXT_INSTALLMODE_ALLUSERS
${LangFileString} MULTIUSER_INNERTEXT_INSTALLMODE_CURRENTUSER
!endif

I added them in the attached patch and the Japanese translator of the GPLed program "LyX" translated the messages. He requested that I add this translation in my installer but I think that all NSIS users should benefit from this.