You're right! We don't have es-MX. And we might have the current one mislabeled (es-LA)? Feel free to post it anywhere we can grab it. Here's the template: https://goo.gl/HJFacj
If you are planning on adding more languages that would be awesome... The they would love to have the site split and diced up by location please try to keep us all together.
Yes, please improve the translation if you can! If you know it's specific for Mexico we can add es-MX. If it's corrections for es-LA (Latin America) we can modify that translation.