Witchcraft works because of the ability of the practitioner to raise, focus, and send forth energy for a purpose. The language doesn't matter at all and it could be argued that if the practitioner uses language unfamiliar to her/him it might actually impede the process.
Spell craft is very misunderstood by many people because they don't understand that the spell itself has no power. It takes most people years of practice and study to learn spell craft, and the tools, "ingredients," etc. are simply there to help us focus our intention and to act as sympathetic magical correspondences to aid us in our attempts.
The language is basically something you choose while casting your spells. I'd also like to add that in some spells you do not have to chant anything, simply manipulate and direct your energy for a purpose, which is the basic form of spell craft anyway.
I recommend to have this language as a language you feel comfortable with, because it helps you to focus your energy and manipulate it way better. Do not choose a language just because it seems 'cool' or because it's ancient. You better focus on energy manipulation and proper spell casting than having a cool chant or language.
I also recommend to know the language or at least what you're chanting, because you need to focus on something for a purpose and have an obvious intent.