You are being too pessimistic.
Yes, it's hard to figure out cow just from a big corpus; though you might learn enough about 'cow' to form meaningful sentences and answer questions. That's what GPT-3 is doing after all, and language models are still improving quickly.
And we do have context! Just knowing that a language is used by humans gives lots and lots of context. Human cultures and languages have more in common than you might realize.
For example, have a look at Deixis (https://en.wikipedia.org/wiki/Deixis):
> In linguistics, deixis (/ˈdaɪksɪs/, /ˈdeɪksɪs/)[1] is the use of general words and phrases to refer to a specific time, place, or person in context, e.g., the words tomorrow, there, and they. Words are deictic if their semantic meaning is fixed but their denoted meaning varies depending on time and/or place. Words or phrases that require contextual information to be fully understood—for example, English pronouns—are deictic. Deixis is closely related to anaphora. Although this article deals primarily with deixis in spoken language, the concept is sometimes applied to written language, gestures, and communication media as well. In linguistic anthropology, deixis is treated as a particular subclass of the more general semiotic phenomenon of indexicality, a sign "pointing to" some aspect of its context of occurrence.
> Although this article draws examples primarily from English, deixis is believed to be a feature (to some degree) of all natural languages.
I suspect with a large enough corpus you can most likely figure out the deixis of the given language.
See https://linguistics.stackexchange.com/a/4046 for some more structure shared with all human languages:
> However, all languages have some sort of a clause-type thing allowing them to express predication, attribution, etc. See Dixon's Basic Linguistic Theory: Basic Linguistic Theory Volume 1: Methodology . All languages also must have means of expressing cohesion and coherence (texture) although this is much less studied in cross linguistic perspective. Punctuated sentences are a kind of cohesive device.
(A comment on this answer points out that the reality is probabilistic, of course:)
> While I agree with you, it seems to me that linguists who study languages with a strong written tradition often talk about 'sentences', even when their examples are not from writing. Those of us who work on previously unwritten languages (I think) tend to talk about 'texts' or 'utterances'. I think this is an important issue as it relates to how some linguists have ignored the true messiness that is often found in examples of human language.
Of course, all of this refers to natural human languages. Not totally arbitrary constructs.