I looked at some of these sites when I wanted to get some assignments edited before handing them in for my first year at University. I found the best editing service was Oxbridge Editing - the website is really clear and you can check all the different options and different prices available and get a free example before you decide whether you want to use them or not. It was quite expensive but it seemed much more professional than many of the other sites and it was really worth it.

They edited my essays really well, made the language just sound much better and took out all my grammar mistakes so I expect it'd be really useful for you for your language problems. I guess tips to look out for are just make sure they are really making all your spelling and grammar correct and really listening to everything you say you need - it's a good service because you can talk to them on the phone and explain exactly what you want them to do not like some of the others where it looks like it's all done by e-mail and you can't get in touch with them easily.