Well I reply myself :) I'd thought it was a fvwm2 problem because I tried fvwm1, twm, fluxbox and they rendered the right characters. But today I've upgraded to Slackware 14 and the problem I had with bitmap fonts and utf-8 despaired. I still ignore the cause (freetype, xorg?).
Thanks Thomas and the rest of fvwm develpers for your work!
Walter
