What I think helps most with a spell is focusing on the intent, not on the words, and not many people are fluent in Latin anymore. By using Latin, your brain isn't making the direct and immediate connection with the words that it would in say, English or whatever language you are fluent in. By using Latin you are helping yourself to detach and to feed energy into the spell and the intent... instead of "jee I really want to make it rain, it has to work..."
Am I making sense, lol? Sometimes it helps to turn a spell into gibberish, like: "I will it to rain" would become "Wit ranii litlo" by switching up the letters. You become more connected with the power than the words.
I don't think any one language is more powerful than another, it is what YOU make powerful.