[gecko]

French spelling is unidirectional. I can comfortably say any French word (albeit with a couple key exceptions, such as "et" or "clef", that break rules), but I can't reliably go from someone talking to how to spell it. "Eaux", "eau", "au", and "aux", or alternatively e.g. "ou", "oux", etc., all have identical pronunciations, but different spellings.

Unsurprisingly, we can vaguely quantify this by looking at dyslexia amongst languages. English and various Southeast Asian languages that rely on Chinese ideographs are by far the worst, followed by things like Arabic, French, Hebrew, and German that have fewer exceptions but less guidance, and then followed last by things like Spanish, Cherokee, and so on that are truly one-to-one.