Actually, this is supported. Polish version of FS had additional characters for the translation. Unfortunately, it causes slight problems with some custom campaigns, for example in Procyon Insurgency I've had an object named "Unknown" and another, named "Unknownó". Normally, FS wouldn't show "ó", because it's not in the standard font, but I've had Polish fonts which do have this character. So, additional characters work. There's a font creation tool for FS somewhere, so I don't see why other alphabets shouldn't be doable (even the ones like Cyrillic or Chinese). Also, there was a Japanese translation for FS somewhere, but it ran on a non-SCP, non-retail version of FS.