Having had a night to think about our discussion yesterday, I realized my argument wasn't as precise as it should have been.
Let's start again with the statement "Lee Harvey Oswald killed John F. Kennedy".
In the thread you referred to, I wanted to make clear that the question whether it is a proposition depends on the context.
Before, October 18, 1939, the day Lee Harvey Oswald was born, the statement is clearly not a proposition, since one of the terms in the statement is undefined.
Unlike you say, I don't subscribe to the notion that a proposition can be meaningless. A statement that has the structural form of a proposition, but one of its referents is undefined, is not a propositions. Propositions in classical logic (and ORM is based on classical logic), unlike values in a relational database have only two possible values: true or false. In SQL terms: propositions are NOT NULL.
Between the day Lee Harvey Oswald was born and the event in Dallas, November 22, 1963, the statement "Lee Harvey Oswald killed John F. Kennedy", is a proposition. Both referents Lee Harvey Oswald and John F. Kennedy are defined (and let's for arguments sake assume that there is no ambiguity as to whom we are referring to). Let's also assume that the verb "to kill" is well defined, making the statement "Lee Harvey Oswald killed John F. Kennedy" a valid proposition.
Its truth value however is false. Between October 18, 1939 and November 22, 1963 John F. Kennedy was alive, so he could not have been killed.
This is where I'd like to correct my earlier argument. Even though the statement "Lee Harvey Oswald killed John F. Kennedy" is a proposition when stated between October 18, 1939 and November 22, 1963, it is not a fact.
Facts are propositions that are held to be true.
The statement "Lee Harvey Oswald killed John F. Kennedy" is only a fact after November 22, 1963.
Now that I have rectified my original argument, let me address your message.
"A fact is a proposition that is either true, false or meaningless"
No, as I just explained a fact is a proposition that is true. Meaningless propositions don't exist.
"Propositions do not appear out of thin air - they are made by people."
Although propositions do not appear out of thin air, they don't necessarily have to be made by people. Two interacting theorem provers may well exchange propositions that have never been conceived by humans.
The focus on people smacks a bit like insisting that consciousness is somehow necessary in the Copenhagen interpretation of quantum mechanics.
How a proposition comes about is irrelevant for its truth value, while the context in which a proposition is used may change its truth value, or even render a statement that is considered a proposition in another context into something that is not considered a proposition.
"When a person makes a proposition, the person must use a language to express the proposition."
While this is true, it also applies to non-humans that create propositions, also the language a proposition is stated in is irrelevant for the truth value of the proposition.
Under every condition "Lee Harvey Oswald killed John F. Kennedy" behaves exactly like "Lee Harvey Oswald vermoordde John F. Kennedy". Each statement is meaningless and therefore not a proposition before October 18, 1939. Each statement is a preposition after that date, with false as its truth value until 22 November 1963, after that date both prepositions are held to be true (at least to the extent that people believe the official reading of the case).
Since we can't distinguish "Lee Harvey Oswald killed John F. Kennedy" from "Lee Harvey Oswald vermoordde John F. Kennedy" on any logical ground, they are equivalent. There are no two propositions, there is only one, it is just uttered in two different languages.
On top of that, none of us was there as an observer (I presume), so it's all hearsay anyway.