Dresden Files: New York
Art of Spellcasting
Spellcraft is different from innate powers, such as speaking with the dead, precognition, empathy, etc. Think of spellcraft as a toolset, rather than as a single tool. When crafting the spell you need for the job, you’re accessing what you need from the toolset and giving it power from a wide variety of sources. An innate power, on the other hand, is a single tool used for only a small sub-set of tasks and it draws its power from a single source that never changes. It’s the difference between a carpenter’s workshop and a hammer.
Most spellcasters have some sort of spoken component to their magic—activating words, ritual chants, and so on. These words are always in a language that isn’t their native one. If a wizard casts spells using words he commonly uses or hears in a context other than casting spells, there’s too much risk of his magical power finding expression every single time he says something, potentially creating a nasty side-effect. Putting the spells in a different language acts as a form of mental insulation, making sure that he doesn’t accidentally loose power at a target (or fry his own mind) every time he swears.