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.
If you translate a spell it might be even better, youwould direct the energy better if you use a translated spell, even if it sounds a little weird, not all spells need to sound like poems.
Lets not forget you can write you own spells, sometimes it would be even a better thing to do than translate another spell or use a spell which was written by another person.
I would read about spell craft in Google, and about writing your own spells.