For an unknown reason the Portuguese.nsh file of NSIS 3 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 Portuguese translator Georger Araujo of the GPLed program "LyX" translated the messages. He agreed to send you this patch.
(He will also come up with another patch ffor Brazilian the next days.)