The language of these communities is not bad, just different. All human languages & dialects have the same level of complexity. For instance, Appalachian dialects often have more irregular past tense verbs than standard English, often preserving archaic past tense forms lost elsewhere. Similarly, African-American has a more complex verbal aspectual system than Standard English (although not quite as complex as West African languages).
The reason people think these dialects are "bad" is because of social conditioning, which is very strong and is attested in almost all civilizations. Speakers of the "prestige" or "standard" dialect usually think speakers of non-standard dialects are uncouth and stupid, while speakers of non-standard dialects can perceive the standard dialect as pretentious. The "Ebonics" debate of ca. 1995 brought out how emotional people on both sides of the issue were.
Even though hearing people talk about "bad grammar" drives linguists crazy, most of us still feel & obey the principles of the conditioning. I mean, like, you won't be seeing me use no double negatives in them academic papers!
Languages don't degenerate - they evolve. Since human languages were first recorded, all languages have shown the same level of complexity, even those which were not written down until the 20th century. English is not getting worse, and the cave people of 40,000 BC did not speak in primitive grunts.
People think language "degenerates" for two reasons. One is that the grammar of a particular individual is relatively stable over a lifetime. The change usually occurs between generations when children learn the language and are heavily influenced by other speakers of the same age (hence "slang"). A change in a younger generation will be perceived by an older speaker as "degenerate" or "wrong."
The other factor is that languages may simplify one portion of the grammar while making another portion more complex - and for some reason, we only notice the simplification. For instance Old English, used to change noun endings based on its grammatical role in the sentence (subject, direct object, possessive, after prepositions). Modern English now only adds 's onto possessives, BUT has increased its vocabulary to accommodate foreign borrowing, scientific terminology and new marketing products. Modern English may have the largest vocabulary ever - something had to give!
The important point is that no matter what historical stage a human language is in, it has the same level of complexity as in other eras. Even if a portion of the grammar is simplified, another becomes more complex.
I had an interesting conversation about how I would rate the "expressive" quality of a language. The answer is that as a linguist, I don't. Why? Because one person's "pure poetry" is another person's "pure pain." Whether or not one language is "prettier" than another or whether one language expresses itself better is very dependent on personal taste and life experience. I think the best comment on how language taste functions is the following - "People who don't like the sound of German have never heard Marlene Dietrich speak it."
NOPE - The minds of children are "wired" to acquire language automatically (undergoing exactly the same stages, no matter what the language is). From ages 1-5, kids can acquire any possible language (from English to Chinese to Hawaiian), and acquiring multiple languages is no problem, provided children have enough exposure. From 5-10, kids still have an easy time, but once adolescence hits, most people lose the ability to pick up languages easily. Ironically, that is just when most U.S. schools start offering foreign language courses.
During the language acquisition process, children may confuse the two languages, and bilingual speakers may switch between them all their lives (that's called code switching), but these speakers in fact DO have competency in both languages.
On an interesting side note, pop singer Marc Anthony is bilingual in both Spanish and English even though he has lived in New York City most of his life. How did he do it? Anthony told interviewers that his Puerto Rican parents designated Spanish days in the house, where English was not allowed. ¡Viva la differencía!
The answer to this question focuses on how children learn to speak in the first place. This is usually not a subject most people think about until they hear their children using "bad grammar." The fact is that the process of language learning or, as linguists call it, "language acquisition" is different from other types of learning such as learning to tie shoes or add and subtract. These processes must usually be taught by adults to children in order for them to learn it.
Language acquisition, on the other hand, is an automated process in which children (starting at infancy and ending at about the age of 3-4), process the speech they hear from adults and construct their own internal grammar of the language. When the internal process is done, the language has been acquired.
No matter what language a child is acquiring, children must go through the same stages including babbling, one-word, and two-word. Both sounds and sentence structures must be learned in approximately the same order depending on complexity or "markedness." Since the acquisition process is the same for all languages from Chinese to English to Hawaiian, linguists assume that their grammars must conform to a universal set of constraints, making them equal in complexity.
Because the process is automated, parents are not well able to overtly correct their children. When they try, children are typically confused and ignore the correction, but months later parents will notice that the error has been fixed.
Of course, after children have acquired the language, parents can suggest minor corrections, but not all of them may stick.
There is a tendency for non-linguists to speak of non-linguistic symbolic systems such as color meaning (e.g. red+green = Christmas), dreams, music, literary symbols (a circle="unity") as "languages." Although, they share properties as spoken language, there are fundamental differences. The range of ideas that can be expressed is different, and often more limited than in human language.
We can use color or music to set a mood, designate a event (a wedding) or indicate identity (this is the British queen), but we really can't use either system to call up our buddies and ask what time the party is. You need support from spoken or written language for that. There are interesting cognitive properties to be explored in these symbolic systems, but they are NOT the same as of human language that linguists generally study.
When recently asked "Do I speak GAME BOY?" all I could think was "No, but I speak BUFFY (The Vampire Slayer)." Actually both the U.S. Game Boy generation and myself speak English, but we do have different vocabularies from other people reflecting our experiences with gaming and watching cult television. Our world views and thinking processes may even be slightly different, but we still express it in some form of English. The changes in English that happen in younger generations use the same processes that have happened in Latin, Byzantine Greek and Old English. It's just new to us!
One non-spoken form of communication which is a language is sign language. With sign language, you CAN ask where the party is, convey any business, personal or technical information and even whisper (hands lowered). American Sign Language is not a straight translation of English, but has its own system with its own verbal inflections and noun forms. Sadly, hearing impaired individuals who suffer strokes or other neurological disfunctions suffer the same range of speech disorders as other people.
One of the great trick questions in linguistics. What we call Chinese "dialects" are actually separate languages. Two of the major languages are Mandarin, the official spoken language of China, and Cantonese, centered in the South, notably in Hong Kong. Some Chinese-Americans have learned one of the non-official languages (or "dialect") from their family and cannot speak or understand Mandarin at all.
On the flip side, some "languages", such as Swedish, Danish, and Norwegian are actually dialects of each other, understandable by native speakers. Although there are regional differences, they are somewhat like the differences between American and British English (although some linguists argue that Scots English should be properly classified as a separate language!) Political situations such as the one in China and Scandinavia have led to the famous saying "A language is a dialect with an army."
Recently, I saw a message on an internationalization Listserv which declared that Urdu was centuries older than Hindi. Not so at all. Hindi and Urdu are actually the same language, sometimes called "Hindi-Urdu". The main difference is what script they are written in.
Urdu is written in an Arabic script which is typically used by Muslim speakers in Pakistan and India. Hindi is written in Devanagari which is typically used by Hindu speakers of India. It does so happen that this language was first written in an Arabic script and called "Urdu" (as pointed out on the same Listserv). But don't be fooled - a script is not the same thing as a language. Hindi and Urdu are the same language, so they're the same age.
Incidentally, the same phenomenon can be found in the former Yugoslavia where "Serbo-Croatian" is now being "split" into "Serbian (Cyrillic alphabet)" and "Croatian (Roman alphabet)." But underneath it all, they're still close dialects of one language.
In my opinion, we don't know the answer to this question, although some people will give one anyway. Here are some criteria people use, and reasons why linguists don't think they really work.
Oldest Written Form
Some people base their answer on which language got written down first. If you're counting absolute oldest, probably Sumerian or Egyptian wins because they developed a writing system first (both start appearing in about 3200 BC). If you're counting surviving languages, Chinese is often cited (first written in 1500 BC), but Greek is a possible tie because it was written in Linear B beginning ca. 1500 BC.*
*Data from "Ancient Scripts of the World" (http://www.ancientscripts.com)
But all of this is irrelevant, because writing is not equal to speaking.
In 3200 BC, there were many, many languages spoken besides Sumerian and Egyptian, but they weren't fortunate enough to have a writing system. These languages are just as old. To take one interesting case, the Albanian language (spoken north of Greece) was not written down until about the 15th century AD, yet Ptolemy mentions the people in the first century BC.* The linguistic and archaeological evidence suggests that Albanians were a distinct people for even longer than that. So Albanian has probably existed for several millennia, but has only been written down for 500 years. With a twist of fate, Albanian might be considered very "old" and Greek pretty "new".
*See An Introduction to the Indo-European Languages by Philip Baldi.
Longest in the Region
Another criteria people use is how long a language has been spoken in a particular region. For instance, Basque is considered very old because the evidence is that there have been Basque speakers in Spain and France since at least the 2nd century BC and probably longer than that. Similary, Welsh is considered the "oldest language in Britain" because its speakers were there first.
But population movements cannot determine a language's age.
English speakers have moved all over the world, but even if English only arrived on a continent in the 19th century, it does not negate the fact that some form of English was spoken in the 6th century AD in England. Even Welsh has moved a bit, establishing foot holds in Patagonia (Argentina) and Canada - however, this language still originated in Britain.
Age of Sister Languages
Many linguists do date languages to when they split from their parent tongue. For instance, French and Spanish are both descended from Latin, so their age is determined by when they evolved into separate languages (between 400-700 AD). Some languages like Greek and Basque are considered older because they never "split" into daughter languages (although both have dialects), and so maintain their status as a "language." In that criteria, there may be a language with the world record of being spoken the longest without having spawned daughter languages - but no one could tell you which one it is.
Even with this criteria, the situation is still murky. It's true that there was a spoken form of Greek in 1500 BC during the Bronze Age, but if a Bronze Age Greek was transported to Modern Athens, he or she would probably not be able to understand Modern Greek. Even speakers of Classical Greek (500 BC) are lost in Athens unless they have also learned Modern Greek. Speakers of Modern English have trouble with Shakespeare from just 500 years ago.
Languages are continuously evolving over time, and probably most languages, even conservative ones, require special training in order for modern speakers to fully understand older texts. In the final analysis, most modern languages are equally young.
Here's a Jeopardy question (paraphrased) that goofed on a crucial detail, showing that even these experts need a little linguistics tutorial. The answer is supposed to be English mead (an archaic alcoholic beverage made from fermented honey) which is related to the Sanskrit root for 'honey', medh.
But from the way the question is phrased, the implication is that English borrowed the word directly from Sanskrit (most Sanskrit words were borrowed in the 19th-20th centuries) and changed to English mead. What really happened is that both Sanskrit and English inherited mead/medh from their parent langauge, Proto-Indo-European, which was spoken in about 4000 BC. It's a small slip, but one that throws off the timeline by six millenia!
In case you're wondering, Proto-Indo-European is the name for the hypothetical language that spawned most of the languages of Europe and India. Descendants include English, Sanskrit, Russian, Spanish, French, German, Italian, Greek, Serbo-Croatian, Lithuanian, Polish, Czech, Persian, Hindi, Bengali, Gaelic, Welsh, Swedish, Danish, Norwiegian, Albanian, Armenian, and more!
Not all languages are Indo-European though. Famous exceptions include Finnish, Hungarian, Estonian, Basque, Hebrew, Arabic, Turkish, and, of course, the indigenous languages of Africa, East Asia, the Americas, and the Pacific islands.