Your reasoning doesn't work if the language in question really doesn't have a word for 'left' or 'right'. An explanation as to why an argument isn't a proof do

Your reasoning doesn't work if the language in question really doesn't have a word for 'left' or 'right'. An explanation as to why an argument isn't a proof doesn't "not work" simply because the statement being argued happens to be true. GP suggests that a particular argument is used to justify the claim that some language lacks some term, and he points out that that argument does not prove the claim.