I suggest this to debug this problem:
- install (reinstall) a fresh copy of some theme which uses Nokia Sans. For example you can get the original NSeries via terminal by typgin:
sudo gainroot
apt-get install --reinstall hildon-theme-alpha

- switch to the fresh theme you just installed

If you still see some other font used than Nokia Sans, then I suspect you have messed the Nokia Sans font file (yeah, the actual file). To fix that, you need to get these files from other device (or reflash yours):
/opt/usr/share/fonts/nokia/nosnb.ttf
/opt/usr/share/fonts/nokia/nosnr.ttf
/opt/usr/share/fonts/nokia/nosns.ttf
/opt/usr/share/fonts/nokia/nscnr.ttf

If still no go, make a backup and do a reflash.