Magical Latin

[potioncat]

No: HPFGUIDX 186102

 
> Kemper now:
> An even simpler answer would be tradition.  
> As there is at least one spell outside of a Latin root, it would seem that Latin isn't necessary for a spell to work effectively.  Avada Kedavra seems potent and serviceable enough to be getting on with. 
> 

Potioncat:
It's also tradition in our world that magicians use some sort of ancient or unknown phrase to make their magic work. Just about any birthday party magician will ask the party-goers to "Say the magic words_________" before doing some special trick. (hocus pocus, abracadabra, etc)

So JKR mainly used Latin, ancient yet familiar, and not well known. (At least, not well known in the US) So it gives the air of being magical, and lets some of us work out its meaning. It also gave her some framework for creating her spells.