Adequacy of language

What does it mean to say that language M is at least as "adequate" as language L? One option would be to say that any proposition that L can express is a proposition that M can express. This, I think, is too strong a requirement. For sometimes one language cannot express exactly the same proposition as another language does, but in some sense loses nothing thereby in adequacy, or at least the language is at least as good for theology, ethics, science and ordinary life (that's what I mean by "practically"!) For instance, suppose that L is English and M is a restriction of English to those sentences that end with the conjunct "and each thing is identical to itself". Then M cannot express the proposition that there are horses. But the speakers of M do just as well with respect of theology, ethics, science and ordinary life any way: M is just as good as L for theology, ethics, science and ordinary life. Where a speaker of L would say that there are horses, the speaker of M will say that there are horses and each thing is identical to itself.

I do not have a clear notion of "adequacy" here—suggestions are welcome. Here, for what it is worth, are two interesting examples of how the notion might be useful.

1. Detensing: It is well known that tensed sentences like "I am now in pain" cannot be translated into token-reflexive sentences like "My pain is simultaneous with this utterance" (for instance, the latter sentence entails the occurrence of an utterance). But perhaps one can say, more weakly, that a tenseless language that makes use of token-reflexive forms like this is just as adequate as the tensed language. Certainly, it is just as adequate for ethics, science, ordinary life and probably theology. Instead of saying a sentence of ethics, science, ordinary life and theology like "It is now time for me to partially fulfill my duty of thanking God for the nomic orderliness of the universe", we just say: "This utterance is simultaneous with the time for the partial fulfillment of my duty of thanking God for the nomic orderliness of the universe." In saying this, we are saying something different. Different, yes, but in practice just as useful for ethhics, science, ordinary life and theology.

2. Indicatives: Maybe

1. "If the Queen visits me today, I will be prepared"

does not just mean

2. "I will be prepared for the Queen's visit or the Queen won't visit me today or both."

Nor does it just mean

3. "P(I will be prepared | the Queen visits me today) is high"

(claim (3) does not give modus ponens). In fact, plausibly, there is no paraphrase of the indicative conditional except in terms of indicative conditionals (including ones involving "unless" and other variants). Fine. But one can still say that all indicative conditionals could be dropped from English, and the resulting language would be just as adequate. I would not be saying the same thing as (1) if I affirmed the conjunction of (2) and (3), but I would lose nothing by doing so.