it's better if you understand it. you need to feel confident while saying it. if you don't know how to pronounce a word, you'll concentrate more on how to say it or what it means instead of the actual spell. same goes for rhyme, a spell doesn't have to, if you feel better using a rhyming chant, then go for it, but if you feel goofie, then go for something else. while the words are important, you need to say them with conviction. so one final point, be sure you choose your words wisely. a common example is when calling upon dragons, don't say something like 'i want you to burn away my enemies' because they will think you seriously mean set them on fire, when you really mean keep your enemies from getting in your way. so watch your wording.
the most common, maybe because i'm English, but English [Elizabethan most of the time] but Latin is another one i do come across a lot, Gaelic, Egyptian, seen a couple written in other languages, but i think it's because the people speak said language so they wrote it in their native tongue.