I believe any dead language is powerful for spell work. I use Latin, simply because it is the easiest for me to pronounce, but as my understanding of dead languages increases, I am sure I will add others to my practice. I have had far more success with my Latin spell work than when using English.
When we are doing spell work, we need to tap into the sub-conscious. It is our sub-conscious mind that goes about the work of materializing our desires. So I keep my spells limited to a few power words or thoughts. I don’t work with complex sentence structures. This allows the sub-conscious to hone in on the power word/thought with ease. By using a language we don’t use in day to day speech, we are disengaging our ego mind and “letting go”. This detachment is absolutely necessary for any manifestation. When we are attached, we are blocking.
My formula, for a super charged spell, is to keep it short, simple, and in a language my ego mind doesn’t understand.