I'm currently creating trying to create a relationship between two models but am facing the following error. It's strange because I've done this many times before without issue. Could it be related to namespaces?
Fuel\Core\PhpErrorException [ Runtime Recoverable error ]: Object of class Language\Model_language could not be converted to string
Your classes are named incorrect. There should be a capital after the underscore, so Model_Language, and Model_User.
And when specifying relations, you should not use a leading backslash when specifying fully namespaced classnames in strings, so "User\Model_User" and "Language\Model_Language".
Also, if User and Language are modules or packages, they should be loaded before the class can be used.
You didn't mention the Fuel version you are using, but it's always good to switch (temporarily) to 1.8/develop to see if you're experiencing a bug that is already solved.
If you still have the issue in 1.8/dev, please create an issue for it at https://github.com/fuel/orm/issues with a documented use case and code samples.