First, I suppose, one must attempt to define "language". A common definition includes the notion that language is an arbitrary and convention system used for communication in a particular community. Arbitrary refers to the fact that we say "apple", Spanish says "manzana" and there is no real relationship between the sounds/words and the meaning. The exception is onomonopia--woof, clang, buzz, and so on. But these are not universal-- dogs 'guau" (sort of like "wow") in Spanish.
Convention is that the community using a language agrees that the sounds/word "apple" refers to that particular red fruit. It's not absolute, the category lines can be fuzzy, but especially with concrete objects we tend to agree on the meaning.
There are no "primitive" languages. All languages that we know of are developed and capable of expressing the needs of their speakers.
There is no such thing as "bad grammar". Grammar, per linguists, is the entire system of language including phonology, morphology, syntax, semantics, and pragmatics. Syntax is the construction of utterances, sentences. When I tell someone I teach English, they sometimes tell me they have bad grammar. By that they mean they use nonstandard forms or constructions.
Nonstandard and standard are attempts to be judgement free in assessing language. "Ain't" is indeed a word. It is considered nonstandard, however. Standard language is easy to define in the abstract and virtually impossible to define in concrete details. Basically, standard language is that spoken by educated native speakers. Or perhaps, spoken by speakers with power. When a president repeatedly uses "to impact" vs "to have an impact on", it may grow in popularity and become an accepted part of the language.
Descriptive linguists observe the usage, the changes, the acceptability, and describe the who, how, when, why, etc. Languages change. This is neither good nor bad; it is simply an attribute of language. All languages will change over time and often over distance.
A prescriptive view says that we should never say "ain't", we should always differentiate lay and lie, and we judge you on what you say in reference to some idealized idea of language that is typically traditional. And arbitrary, actually. If you fail to speak accordingly, your grammar is "bad" and by extension, you are bad.
While I am using English as my examples, the same judgmentalism occurs in other languages as well. Some countries/languages have language academies that attempt to regulate or dictate what words and constructions are acceptable. The Académie française averred that the use of Anglicisms, such as "le week-end" should not be used--French has a perfectly good way to say "fin de samaine". Of course, the Anglicism carries additional meaning--the speaker knows some English, the speaker is hip and modern, etc. And legislating language is an exercise in futility. People own their language and people decide how they will speak.
Linguists, both theoretical and applied, typically are descriptivists. We are interested in how people speak, how they choose constructions and vocabulary, how they shift depending on topic and audience. And we describe these. If asked whether a construction is good or bad, we would reply with something akin to "that is considered a nonstandard form but is commonly used in informal language with peers". As second language teachers we want to let learners know the contexts of communication, not merely words or syntax.
I, personally, as a descriptivist, love hearing and using unusual forms or vocabulary. In vocab it's often slang, which I find highly creative. I don't often teach slang--it changes quickly, depends on who is talking, and is often picked up naturally by learners when they interact with native speaking peers. In syntax, I deliberately make comments like "can you reach me down that glass?". (Obviously I don't teach this to English learners!). Or, as a transfer from Spanish, "will you gift me that cookie?" (though I have heard this from native English speakers and speculate that it will become widespread one day).
So we should pay attention primarily to what meaning people are expressing. But if we notice forms or pronunciations we find different, we need to have a descriptive attitude of curiosity. Why did they use that, in this setting, on that topic? rather than "They should have said x; since they didn't, they are probably not very smart".
Judging language is judging people.....and there is more to a person than their language.
note: back in time, "their" was used in sentences like "Each student should bring their book to class". In the 20th century, this was deemed wrong because the subject is singular and the possessive is plural. Today, is it not just as wrong to choose his or her when you don't know the identity of the speaker? "Their" is back, hallelujah!
second note: "Talk like this"--what does that even mean? Why should we proscribe how people choose to speak? And what's this about Speak English! If we have freedom of speech, doesn't that mean we have freedom to choose what language we speak in?