What does my words are beyond doubt or language mean? I’m not familiar with the phrase “beyond doubt or language.”
It's a poetic way to mean that when the Knight speaks it is impossible to doubts their words: this could mean that people trust that they are true and correct, but might also mean that it's impossible to misunderstand them.
Also, when the Knight speaks the language barrier is no issue. Their meaning is always conveyed, one way or another.