Not sure about the best software; I have installed gnome-subtitles and
it is OK. I particularly like it's ability to have two files
side-by-side, frame-by-frame, to translate.
http://gnome-subtitles.sourceforge.net/
--A
From what I was reading on the net when trying different subtitle
editors for Windows Miyu (http://www.fluffalopefactory.com/miyu/) for
Mac OS had good reviews. Haven't tried it though.
Regards,
Sergey Khenkin