How do you spell that

This is why English is obsolete, in Finnish language you pronounce things way you write them.
In English, sometimes you spell things how you pronounce them, and other times you spell them differently. It depends on which language each word came from, and how much it changed since then. There are German words, Latin words, French words, Gaelic words, Greek words, even a couple of Hungarian words, and who knows how many from various other sources. In this case, we're talking about a Latin phrase.
As one quotation puts it, "The English Language doesn't "borrow" words from other languages, it follows them down dark alleys, knocks them over the head, and ransacks their pockets for loose grammar."
Having clear rules for language and spelling is a good thing, so English has several different rules for each of them. More is better, right? Forming a plural is dependent on the source of the word and several other irregular factors, so the plural of boy is boys (add an "s"), the plural of man is men (change the vowel), the plural of fish can be either fish (no change) or fishes (add "es") if there is more than one type, and the plural of octopus is octopi (change the "us" to "i"). A fine example of a rule in English spelling is "I before E, except after C.....unless it's weird". We even need to have exceptions to the exceptions....except if....