Language has rules, not just signals.

to speak a language is to participate in a rule-governed social game (Wittgenstein).
solipsism would have to explain where the rule structure comes from.

language involves shared norms; norms require more than one subject; therefore language presupposes at least the concept of other minds.

if solipsism were true you’d have to invent:

• a whole normative system

• with self-binding constraints

• for no reason whatsoever

that is: the very shape and function of language presupposes plurality.