Yes, that's true about natural language borrowing, to some extent. Note that calques (borrowing of a phrase with the vocabulary translated but the syntactic structure of the source language retained) are also common; presumably the artificial language would want to avoid these.
Also, some very high percentage of natural language borrowings are nouns. This clearly has a lot to do with the fact that if you encounter a new object, a natural way to label it is to adopt the existing term of the people who've already encountered it, but I think there are other factors: I reckon it would be fair to say that nouns are syntactically less complex than verbs. I suspect you'd encounter least trouble importing concrete nouns into your artificial language.
It's interesting that you talk about the phonetics of the artificial language; I realised that I've been imagining something entirely text-based, but of course there's no reason it should be (except for the practical/technical difficulties with processing acoustic input, I guess).
I'm curious what got missed off at the end of your post?
Oops, I want back to edit and forgot to write the rest of that paragraph.
I was going to say, supporting borrowing means you need to retain all the borrowable word forms - nouns, adjectives, and verbs - which rules out some extra-radical possibilities, like making a language where verbs are a closed set and actions are represented by nouns. But to my knowledge no natural languages do that, so that's not much of a restriction.
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.