The origanal language of a spell depends on the spell creators veiw, and tradition.
Many used the germanic runes for spells in written forms, many spoke in Italian, or Latin for spells.
Depending on the religion, and century, spells were written in different forms of letters, or words, even pictures, to keep the spell, or ritual;secret. In the witch hunts, this became way more emphazised on, and is often still used in world wide nations like Europe, today!
The spell words are not important, saying a spell in greek, may actually aid a spell. Why? Because you have to keep focused, and pronounce the words correctly: this then can aid, due to the concentration absorbed.