Some time ago, I translated these too and these came from xml files, that had to be populated with translations at build time.
However, back then intltool didn't even extracted correctly the strings - most of them contained line breaks, but intltool did not included those in the .pot file. This was when I gave up translating xscreensaver, however it is possible that this is not a problem now. But I guess the xml files remain.
Some time ago, I translated these too and these came from xml files, that had to be populated with translations at build time.
However, back then intltool didn't even extracted correctly the strings - most of them contained line breaks, but intltool did not included those in the .pot file. This was when I gave up translating xscreensaver, however it is possible that this is not a problem now. But I guess the xml files remain.