Update: When I asked this question I hadn't considered that most mathematical texts (in the Western tradition) were written in Latin from the [14th?] century until the [19th?] century. This probably influenced the development of the "modern" (post 1850s-ish?) language of proof-writing. Additionally, the use of symbolic logic, particularly quantifiers, is a product of the late 19th/early 20th century, so the development of rules for inserting and manipulating logical formulae in text probably occurred around then.
While a study of the historical development of the language of mathematics isn't a study of the language itself, it is likely to provide important information. As such, this is now also a question about math history.
Also, I found this:
So if there isn't a definitive "Compendium Linguae Mathematicae", at least I there's a good starting point for further research.
I've decided to wear my anthropologist hat today, and it has come to my attention that, from a linguistic standpoint, the language used to write proofs (in English) is not English. Though it shares a high degree of mutual intelligibility with English, the dialect - though I would argue that it is a distinct pidgin language - which mathematicians use has several features which distinguish it from the English language.
For instance, there are distinct grammatical moods which are used in mathematical writing that are not present in English. In the language of mathematics, it is incorrect to introduce a new subject using the declarative, as in "$x$ is..." (actually "$x$ is..." can be used only if $x$ names a kind of thing rather than a specific thing, but this usage distinguishes it from the declarative mood, anyway.) Instead, there is a grammatical mood which superficially resembles the English imperative which must be used to declare a subject prior to its use in a declarative sentence, as in "let $x$ be..." and "suppose that $x$ is...." I say "superficially" because although the English sentence "Let $x$ be [object]." is imperative, the mathematical sentence of the same form does not relay an instruction to the listener; a response of "No, I don't think I will" is nonsensical in mathematics - this is not the case for the corresponding English sentence.
There are many other more-than-just-jargon examples of of mathematics deviating from the English language. There are even features of mathematical language that more closely resemble those of a typed programming language than any natural language, as evidenced by the rules governing the use of keywords like "fix," "choose," "by," and "at."
So, if mathematical English is not English, what is it?
Are there any books, articles, preprints, documentaries, or Reddit threads that discuss the language of mathematics in detail (without assuming prior fluency in mathematics)?
My initial search has turned up shockingly few results, none of them peer-reviewed, and all of them based on an underlying assumption that the language of mathematics is just English with some additional technical vocabulary (though a few webpages highlight that there are syntactic differences without ever addressing them in a systematic way.) If this were true, then native English speakers would be able to pick up proof writing given a week's time with a vocabulary list. Clearly this is not the case; it seems to take students 1 to 2 years to become fluent in mathematics, which is about the length of time that I would expect for a native English speaker to learn Krio.
About the tags: I'm not sure what to tag this with. Strictly speaking, this is a question about linguistics, not mathematics; but the context is so narrow (there are no people whose primary language spoken at home is proof-writing) that it is only relevant within the mathematics community. This might be relevant to mathematics education, since educators (or at the very least, textbook authors) [seemingly] assume that the language of mathematics is inherently understood by native English speakers without introduction; but then I'm not much of an educator. All I know is that within my limited experience as a tutor many students in introductory classes attempt to write proofs in English, only to be told that they "can't say it that way" with little or no explanation. In fact, I only came to the realization that mathematics is a separate language from English when I noticed an ESL student inserting formulae into English sentences in a way that was consistent with the grammar of Mandarin (and grammatically correct, albeit unusual, in English), but mathematically "wrong" in a way that I could not explain at the time.
The reason mathematics is difficult for the untrained to understand is obviously not because of the language but because of the logical reasoning involved. One must have a sufficiently good grasp of basic FOL (semantics and deductive rules) in order to be able to follow mathematical arguments with ease. Ultimately, that dependency on FOL is what makes mathematical writings (especially more rigorous ones) look more like formal languages rather than natural language!
Many educators themselves are woefully ignorant about the issues. For instance, they say "can't say it that way" instead of "it is not logically permissible" because they do not even know what precisely is logically permissible. And most people cannot figure this out on their own; one needs to be taught a deductive system for FOL such as this one. (The situation is the same as with programming; most people cannot construct a programming language all by themselves without knowing any existing programming language.) It also does not help that many educators and mathematicians alike use misleading words like "let" for creating a ∀-subcontext when there actually are appropriate words with the correct meaning like "given [any]" / "take any" (for more explanation see this post).