Maybe, but if we're doing formal proofs then we're not in the realm of opinion anymore. So the point is to substantiate any claim as to what, exactly, is reasonable to call "the mtg language". I really doubt there's any context in which a language can be formally described as "a limited number of texts"... especially if that language is supposed to be Turing complete (which implies non - finite-ness) !
As to the relation between the language and the rules, if you think about it, if the language is not equivalent to the rules, then it can't represent (and, therefore, modify) every rule in the game (as it is supposed to). And if the language and the rules are equivalent, then proving one is Turing complete also proves it for the other. So you can ignore the rules and focus any proof on the language.
BTW, the kind of language the ability text language is, has a name: it's a Controlled Natural Language. [https://en.m.wikipedia.org/wiki/Controlled_natural_language].
As to the relation between the language and the rules, if you think about it, if the language is not equivalent to the rules, then it can't represent (and, therefore, modify) every rule in the game (as it is supposed to). And if the language and the rules are equivalent, then proving one is Turing complete also proves it for the other. So you can ignore the rules and focus any proof on the language.