Lots of plugins exist for human translation (as distinct from automatic machine translation). Here’s a blog post that lists and compares some of the best ones: http://www.wpmayor.com/plugin-reviews/best-wordpress-translation-plugins/
In my personal experience, WordPress Multilingual seems to be the most solid one for a long-term commitment: http://wpml.org. They have a few different price options (including free for non-profits). Regardless their pricing, their support is excellent, and they will work with you to help you when you run into difficulty.