The po files for l10n work fine with xscreensaver's main window but not with
the settings window. Many entries present in the po file don't show as
translated (wrong numbering), like all the descriptions. There are also some
missing entries to be translated.
The pot file needs a complete revision. I'm not a programmer but if there's
anything I can do, ask me.

I tried to respond to this bug some times, hope it works now.
Just clarifying that the intltool-update command (package intltool) updates the
pot file and also the po files.

intltool-update -m
(checks the translatable strings in source and points out if any are missing to
POTFILES.in or POTFILES.skip)

intltool-update -p
(updates the pot file - translation template - according with the files listed
in POTFILES.in)

intltool-update -d LANG
(LANG= language code; updates the po file for the given language)

These commands fix most of the current problem regarding l10n in xscreensaver
but I couldn't figure how to correct the modes' descriptions because their
translation still don't work. I suspect they should be all in one single line
in the xml files (no line breaks) but didn't test that because it would
probably need to build xscreensaver here.

There's an intltool-update command to parse xml files for translatable strings
but it makes no difference.