What is true of languages also holds true of dialects within a language. Occasionally, you may hear it said that some dialect, such as African American Vernacular English (AAVE) or Puerto Rican Spanish is ungrammatical or deficient. In truth, though, these dialects follow internally consistent rules. That is, they have their own consistent grammatical systems, but ones that differ from the grammars of other speakers of English or Spanish.
Sometimes, it is claimed that some thoughts cannot be expressed in a particular language or contrarily that an idea can only be uttered in one language. If true, that would presumably make some languages better than others. But such claims turn out to be hard to substantiate. What does it mean to say that a concept cannot be expressed in a language? Often, people seem to mean that one language has a particular word for a concept that another language lacks. For example, German has the term schadenfreude, which means "taking pleasure in the misfortune of others." Some English speakers borrow this word when they want to express the idea. Does that mean that we can't express the idea in English? The very act of explaining what the word means demonstrates that it is possible to express the idea. True, it may not be always possible to translate an idea word-for-word, but paraphrase and other techniques will get the job done. Languages are flexible enough to adapt and expand to the needs of speakers. And if speakers of a language need a particular concept often enough, they will create a word to express it, either by relying on native word-creation processes or by borrowing the term from another language. Indeed, enough English speakers have found schadenfreude to be a useful term that it can now be found in the larger English dictionaries, although it still has the feel of a foreign word. Over three quarters of the words in Modern English, particularly the more learned terms, are borrowed from other languages.
Other arguments for the intrinsic superiority of one language over another make equally little sense. Language is fundamentally an arbitrary convention. There is no principled reason why the animal that English speakers label dog must be identified with that particular string of sounds. Speakers of other languages get along just fine with entirely different strings of sound: chien in French, perro in Spanish, gae in Korean, naayi in Tamil, and so on. It would be unreasonable to say that one of these words was a more logical fit for the animal.
Similarly, we would laugh if someone asked us which is better, to put your adjectives before your nouns (as English does) or to put them after (as does Spanish). The question is fatuous. The order that each language follows is simply a convention that must be followed if we wish to be understood in that language. Evaluations of better or worse don't enter into the picture.
What holds for individual words and rules of a language holds for the whole collection of words and rules that constitute the language: there is no linguistic basis for declaring one language better than another. For the same reasons, it's impossible to find objective reasons to declare a particular dialect of a language superior to another dialect. This equality of dialects is important to stress because traditional grammar typically values one dialect as proper and denigrates others as inferior corruptions. Labels like "substandard English," which are sometimes used in older works to label certain dialects of English, reflect such attitudes. In this view, correct grammar is an elite property of a few "correct" speakers. When traditional grammarians appeal to usage in order to justify their rules, they do not invoke the general usage of most people. They select a handful of prestigious writers as their models. Linguists try not to privilege the language of one group over another just because that group has the prestige in society. That distinction is social, not linguistic.
Most linguists do accept the practical usefulness of having a standard form, especially in writing, and virtually all conform to the traditional notions of standard English in their professional work. But one can adopt a standard as an arbitrary convenience without bringing along with it elitist assumptions that using it makes you better than those who do not. Rather than conceiving of prescriptive violations as "errors" or "wrong", many linguists speak of sentences as being acceptable or unacceptable. Teachers, for example, will often tell students "ain't is not a word." In a linguistic sense, of course, ain't certainly is a word. Among many groups, however, particularly those with power, it is not a socially acceptable one. That is, a linguist would find a sentence such as
(8) #They ain't coming.
to be perfectly grammatical, but unacceptable in many contexts, such as formal writing, a job interview, etc.
 Tamil is one of the major languages of India and is mostly spoken in southern India and the northern portion of Sri Lanka. It is also spoken by immigrant communities in various countries around the world.