How do you even know which things are part of the self and which are 'external'? That feels arbitrary or a 'I know it when I see it' to me.
You could say the self is things you control but under any scrutiny in almost any domain that's not true for what we think of as 'self' either.
I agree solipsism isn't true, but I don't know that I agree you couldn't doubt the external world without language. I think language is just the mechanism we use to describe our inner thoughts. Math is 'real' and describes the world whether you use base 10 or 12 or don't know about math at all.
How could you convince a solipsist of that? It seems impossible to disprove the position "I am imagining that anything outside my consciousness is real". Anything you cite as evidence is premised on the conclusion.