source page in lang1 on one side showing diff, and
aligned source page in lang2 on the other side
while allowing a dual scroll of the screen, like kdiff3 (KDE), or winmerge (windoze), as example, do.
I recently saw a java soft. for translators which was doing just that, which was what I suggested in wikisym 07 first talk about wiki translation User interface
This feature is a "low priority" one, but kind of important in the mid-long term for the tool to be really helpful on big load sites..., avoinding translators have they eyes really tired looking up and down while attempting to translate updated changes from one lang to any other