The importance to take into account from where the translator from is great. If you are looking for your work to be translated that is. Why? because for instance in Mexico,people in the north side of the Mexican Republic use different words and people on the south side of the mexican republic use a different set of vocabulary to describe the same thing or to talk about the same thing. So therefore, it is in my perspective that in this case it would be best to look for a person that even though they were born on the north, south or even on the central part of the Mexican Republic, it is of great importance for that person to have a broad and educated vocabulary.
You will be able to see these traits when you listen how he or she expresses herself. And therefore, see if he or she is compatible with the piece that you would like to translate. I truly think that the best way to capture the soul of the writer is by using a neutral Spanish to appeals to a wide audience, without worrying about from where your reader is from.
One of the most important things that you should check is their educational background because it will be part of what will help you decide if this person is what you are looking for.
Now another factor is for you to be one hundred percent sure of what you want for your piece and transfer that wish to your translator in a very clear fashion. It is also of great importance that you and your translator communicate with eachother on a regular basis. Remember, while your piece is being translated,your translator is your best friend. Due to the fact that you are entrusting him or her a part of who you are. And on a practical level, your work needs to be reviewed as much as possible, so that you have no doubt that you are fully satisfied with the job that he or she is doing for you. Because if you do not have any contact with your translator, then the final draft of your piece may not be what you wanted it to be.
So make sure that you are open to any questions that your translator may have. This factor is also true when facing any type of translation, it does not only pertain to book translations but to any kind of document.
Always do what is best for your piece! after all, every piece that you write is part of your artistic spark, which is what makes you a unique and unrepeatable artistic being!