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.
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.
Reluctant comes from the Latin roots re- (“against”) and luctari (“to fight”). Reluctance is to fight against what should be done — literally.
From luctari, we also get the Spanish for exactly the same, “to fight.”
But they don’t sound similar. How did luchar evolve?
Interestingly, in most Latin words that had a -ct- sound, this -ct- sound evolved into -ch- as Latin evolved into Spanish. Think about night/noche and eight/octagon. The same pattern explains luctari turning into luchar.
We see this relationship clearly with the l-ct to l-ch mapping between the two.
The Latin for “eight” is Octo, from which we get the English Octagon.
And suspect falls exactly into this pattern: the English s-s-p-ct maps exactly to the Spanish s-s-p-ch.