writing letters in spanish
If you want just a simple translation, you can always go and use the translation device on this website, it's actually quite good compared to some others. If you want to write it by yourself, you might want to go check the reference section of this website
There is a site called Lenguajero that is free. I have read that it may provide a native speaker to evaluate your written Spanish, but it may be limited to a predefined writing topic. You write on the topic and then post to get it evaluated.
You might try it and let us know: lenguajero