After installing the package ttf-xfree86-nonfree, the text in "Luxi Serif" font became "spell checked".
Then I remove and purge the package, and Openoffice.org didn't propose the "Luxi Serif" font anymore (it was the default font previously!). I add a font substitution in menu Tools->Options->Openoffice.org->fonts, to be able to display my old documents which still use this font.
So at least, I don't have any font which can't be spell checked. -- Michaël P.