It, however, reads its strings from an external file for localisation reasons.
Check /usr/share/locale/$LANG/LC_MESSAGES for something like osso-systemui-tklock.mo. (I can't remember the file name exactly, so grep -i for whatever the string "Swipe to unlock" is in your language. Edit: Check pelago's post below)

Once found, either install "gettext" on the N900 (available in the SDK repository; be careful!) or copy the mo file to a Linux computer (or a Windows one, if you can find Windows binaries of gettext).

After that, you can invoke "msgunfmt mofile.mo -o mofile.po" and find the string in the resulting po file. Once found, replace it with a space character and, then, once saved, run "msgfmt pofile.po -o mofile.mo" and copy the final result back to the LC_MESSAGES folder