Rationality requires intelligence, and the kind of intelligence that we use (for communication, progress, FAI, etc.) runs on language.
It seems that the place we should start is optimizing language for intelligence and rationality. One of SIAI's proposals includes using Lojban to interface between humans and an FAI. And of course, I should hope the programming language used to build a FAI would be "rational". But it would seem to me that the human-generated priors, correct epistemic rationality, decision theory, metaethics, etc. all depend on using a language that sufficiently rigorously maps to our territory.
Are "naturally evolved" languages such as English sufficient, with EY-style taboos and neologisms? Or are they sick to the core?
Please forgive and point me towards previous discussion or sequences about this topic.
I agree. However, making something look less scary in the beginning still constitutes an improvement from a pedagogical point of view. The more quickly you can learn the basic morphology and lexicon, the sooner you can begin the process of intuiting the higher-level rules and social conventions that govern larger units of discourse.
Due to a large amount of basic structure common to all human language, it's usually not that hard to learn how to sound grammatical. The difficult part of acquiring a new language is learning how to sound idiomatic. And this basically amounts to learning a new set of social conventions. So there may not be much that language-planning per se can do to facilitate this aspect of language-learning -- which may be a large part of your point. But I would emphasize that the issue here is more sociological than linguistic: it isn't that the structure of the human language apparatus prevents us from creating languages that are easier to learn than existing natural languages -- after all, existing languages are not optimized for ease of learning, especially as second languages. It's just that constructing a grammar is not the same as constructing the conventions and norms of a speech community, and the latter may be a more difficult task.
This kind of drift will presumably happen given enough time, but it's worth noting that (for obvious reasons) Esperantists tend to be more disciplined about maintaining the integrity of the language than is typical among speakers of most languages, and they've been pretty successful so far.
One advantage Esperanto has over natural language, is that nearly all of its speakers speak it as a second language. That is way most of its learners are self-consciously trying to maintain its integrity.