I was going to say the exact same thing...
Yeah actually. You do need to know that in order to assert it. Because there are far too many counter-examples which make your sweeping generalization that "all language is meaningful" obviously false.
If you don't speak Swahili then you do not know if a particular sentence is even IN Swahili, let alone assert if it's meaningful or not. The best you can do is ask a Swahili speaker IF a proposition is meaningful. This "testability" thing science insists on - you can't escape from it.
Language is an ever-evolving system, not a thing. It requires multiple components for it to work. One of those components is SHARED context/knowledge/experience/memory!
Without such a shared context/knowledge/experience/memory the INTERPRETATIVE portion of the system that is language does not work. You can't even understand what I am saying in (what you might suppose is) English! Technical jargon is perfect example.
Is Meoritic script meaningful?
Are the writings of the Harappan civilisation meaningful?
Will Shakespeare's writings still be meaningful to anybody 5000 years from now when all of its context and cultural relevance are long forgotten?
https://www.lesswrong.com/posts/HLqWn5L ... -distances