Chozo languages are very complicated, because they can make a huge variety of sounds- much more than humans are capable of making. Zebesian Chozo is one of the easiest Chozo languages to learn, because they simplified it after encountering many other races. Chozo groups who tended to keep to themselves, like the Tallon IV Chozo, had incredibly complicated written and spoken languages. Samus knows Zebesian Chozo but lacks the vocal chords needed to speak in it, so for convenience, her family would (telepathically) speak to her in her native human language. She finds it easier to read and write in Chozo.