The Spanish mancha (“spot” or “stain”) comes from the Latin for the same, macula.
From the Latin macula, we get the English… immaculate — which literally means (knowing the negation prefix of im-) “without a stain.” So the immaculate conception truly was perfect!
How this sound changed was interesting: often Latin words with a ct- or cl- or other hard letters after a c- sound turn into a suave ch in Spanish. For a distant example, see duct and ducha, or nocturnal and noche. (The ct- is much more common than the cl‑, but they’re cousins!) Thus, we can see the m‑ch of mancha mapping to the (im-)m‑cl of immaculate.
The law and the good, in European languages, are associated with straight lines; the bad with the crooked. Think about the word crooked itself, literally! Or about right/rectangle, or the Greek ortho- for straight, hence, orthodox as well as orthodontics.
This is why it makes sense that Derecho — Spanish for straight and also for law — comes from the same Latin root that gives us direct.
The “ct” in the original direct turned into a “ch” in Spanish, in the usual pattern of “ct” turning into “ch” as Latin grew into Spanish.
The Latin for “eight” is Octo, from which we get the English Octagon.
Since most Latin words with a ‑ct- sound, like Octo, had the ‑ct- turn into a ‑ch- as the language evolved into Spanish, it is no surprise that eight in Spanish is ocho.
This same pattern manifests itself in noche/nocturnal, leche/lactose, and is one of our favorite patterns here at ForNerds!
The Spanish hechizo (“spell”; nothing to do with the letters in words, but what a witch casts on you!) comes from the Latin facticius (“made by art”; “artificial” — indeed, that which is artificial is just something not occurring naturally but instead made by art!).
But how did artificial change to mean “spell”? Think about it this way: casting a spell goes against nature — it’s what the wicked, crazy and profoundly unnatural woman does! Think of the three weird sisters in Macbeth, and how they unnaturally stir up all the elements!
From hechizo (more specifically, from it’s Portuguese twin cognate, feitiço), we get the English fetish. How? Well, you have a fetish when the recipient casts a spell on you to become obsessed with the object of your fetish, right? Enough said!
That root facticius turned into hechizo by changing via two common patterns: the initial F in Latin tended to turn into an H as Latin turned into Spanish (compare fig and higo, or fume and humo!) and the ‑ct- tended to change to a ‑ch- (compare noche and noctural; or ocho and octagon). Thus the h‑ch of hechizo maps to the f‑sh of fetish.
The Spanish Decir (“to say”) comes from the Latin dictio for “word”. Its participle form is dicho — and dicho also means “saying”, in the sense of, a cliche.
Thus decir is another example of the “ct” sound in Latin turning into the “ch” sound in Spanish — and is also related to the English word… dictionary.