Where in the Constitution does it state that its language should be read as inclusive of non-US persons unless explicitly stated otherwise?

The part where it uses the word "person" without a qualifier such as "who is a american citizen"?

Where in the Constitution does it state that its language should be read as exclusive of non-US persons unless explicitly stated otherwise?

It doesn't specify either way, that's the problem. It's not a modern legal document with precise terms defined, it's written in sometimes intentionally vague 18th century prose, so either interpretation is equally valid as far as the text alone is concerned.

If it says "citizen" in one place, and "person" in another where it could as easily have said "citizen", elementary logic dictates that "person" does not imply "citizen". Saying otherwise is a brazen attempt to muddy clear water.

The framers were not stupid. They knew what they were writing. If they had meant "citizen", they knew how to write it.

"The framers weren't stupid lol" isn't a compelling argument.

Show me documentary evidence or legal precedent from the time demonstrating that intent on the part of the framers, or else you're just projecting.

After all, apparently everyone was wrong about what "well regulated militia" actually meant until 2008. So it must not all be as crystal clear and elementary as you claim.

Muddying the water has been a successful strategy for lawyers for millennia. That does not make it less reprehensible to do, nor justify being fooled by it.

There were a lot of lawyers among the framers. They didn't choose the wording of something as important as the First Amendment randomly.

The Constitution binds the US government in all cases. It does not cease to apply when the US government deals with foreigners. If the Constitution places an unqualified restriction on the US government, that restriction applies in all circumstances. The free speech clause does not have any qualification about only applying inside the borders of the US or to citizens.

It would be absurd for the US government to claim that Assange is bound by US law, and can therefore be charged under US statutes, but that the Constitution does not apply to him.