some groups of people don’t use their language properly

From Mapping Applied Linguistics: A Guide for Students and Practitioners by Christopher J. Hall, Patrick H. Smith, and Rachel Wicaksono:

You can substitute several adjectives here for variations on the same myth. Among the most common: some ways of using your language are more beautiful, more complex, more pleasant, more efficient, more logical, more civilized... Many such beliefs arise naturally because of mistrust of ‘the Other’, but in large part language judgements follow from the notion of a ‘standard’ form of the language against which all other varieties can be measured — and found wanting. But in what sense do standard languages exist? They certainly seem to exist in forms of discourse such as newspaper editorials, national language policies and school textbooks. Standard forms of language are appealed to, often when people feel that their national or regional identities or interests are being threatened. Despite the social power of the belief, standard languages don’t exist in the minds of individual speakers; rather, groups of speakers share different degrees of awareness of a set of conventions about what is acceptable, prestigious and desirable. Written language has played perhaps the most important role in ‘fixing’ these conventions as the basis for how others should write and speak.

An extension of this dead end is the belief that some languages are better than others, for example that some are harder or easier to learn, some are closer to God(s), some are more beautiful, more complex, more pleasant, more efficient, more logical, more civilized, etc. Descriptive linguistics and sociolinguistics are useful here to expose the patent nonsense of such beliefs, by comparing the same linguistic unit in different languages or dialects. This allows us to see how the same or a similar element of phonology, for example, can have different linguistic value in different languages, without requiring or entailing any measurement of efficiency, complexity, logic or aesthetics. The /l/ and /r/ sounds of English and many other languages are not differentiated by Chinese speakers, for example, just as the tonal features of Chinese can seem indistinguishable to speakers of atonal languages, such as English. And the ‘illogical’ double negative of many English dialects (‘I ain’t got none’) is part of the ‘standard’ versions of French and Spanish.