This isn't entirely accurate. When foreign words and names are transliterated into Japanese, they are spelled in katakana characters, and since the Japanese don't have all of the sounds necessary to perfectly pronounce most of these words or names, they make due with what they have. This is why the katakana transliteration of 'Aerith' is 'Earisu' (エアリス). The katakana transliteration of 'earth' is 'aasu' (アース). You are correct about the s/th interchangeability however. Because of this, the localizers must've chose to ignore the official romanization of her name and went with 'Aeris' instead."Aeris" is a really roundabout variation on the English word "Earth", referring to her role as the last of "the ancients", a race that's in touch with the spirit of the planet. In Japanese characters, her name is written "Earisu", and when they brought it back to English, they swapped the "A" and the "E". Also note that in Japanese, the character "su" can stand for a trailing "th" (like "Sumisu", for "Smith"), or for a trailing "s" (like "Kurisu" for "Chris"). So that's how we go from "Earth" to "Earsiu" to "Aeris".