Partialy responding to myself:
"Alt+F2" then typing "kdesu kate" works well!
If I well understand, the "things" displayed with "root" privileges are displayed with "smoothed" fonts. With Kate started as described above, the characters is well displayed. Hummmm: the characteres displayed in the TEXTAREA of Firefox 3.6.20 (initial) are well displayed, too.
Sincerly, Patrick