[decimon]

It’s a latin word. The rules of its usage haven’t change in, um, almost two millennia.

That's so of all words derived from Latin?

[EDINVA]

“That’s so of all words derived from Latin?”

No, it is not true of all words DERIVED FROM Latin. It is true for all Latin words, tho more often than not observed in the breach. I.e., forum should be fora, stadium/stadia, etc.