Most Linux distributions ship with "Bitstream Vera" fonts. These are missing most unicode characters in 256 - 256 + 128 range, which is quite trouble if you want to use them for e.g. Czech langauge...
"Classic" solution (e.g. in Qt) is to "borrow" missing glyphs from other fonts.
U++ now contains system that perfoms "glyph composition" - as markings are in fact separately present in the font, it is possible to compose individual characters using base character from ASCII set and some additional glyph. The advantage is that this way, text does not look strange by adding glyphs from another font.
Mirek
P.S.: Hm, now reading this, it sounds like minor issue But I was planning this for quite long and I believe it is critical for U++/Linux... - well, in fact, system is active in Win32 too.