The user_guide module doesn't actually contain the content that you see at http://docs.getfuelcms.com. It just provides the infrastructure to display the user guide and can read other module's documentation including documentation in your application directory. The actual language translation of content would need to happen within the fuel module where the documentation is located at fuel/modules/fuel/views/_docs/. More on it can be read here: http://docs.getfuelcms.com/modules/user_guide
Let me know if you have interest in translating the user guide content.
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http://docs.getfuelcms.com/modules/user_guide
Let me know if you have interest in translating the user guide content.