Non-standard language may be used by an author for other reasons than simply to make character speak in a more realistic way. Non-standard language is incorrect not only from a grammatical point of view, but also from an ethical stance. This puts non-standard language in contrast with standard language, which can be seen as representing the Establishment.
Thus non-standard language can be viewed as a means of protest against society. For example, it can describe an underprivileged social context from the inside, through the very language used. Non-standard language may be seen as a profanity against standard forms even if it is not intrinsically so. As a result, the use of non-standard language may help to make a musical, literary or cinematographic product more saleable to a certain audience (in a provocative, but otherwise meaningless way).
The translation of non-standard language therefore becomes a particularly difficult task. Even when the skilled translator has understood the origin, the cultural background, the social level and the intended effect, the problem remains of how to reproduce the peculiar flavor of a variety of English, how to transfer it to another language.
There's no easy solution. In theory the reader of the translated version should be able to implicitly acquire the same information the reader of the original text acquired. But this is very often just not possible; shades of meaning will almost inevitably get lost in translation, and even if somehow the translator does manage to indicate the precise geographical origin of a character, as well as the character's position in that society, it may well be of little significance to a reader in a foreign language with a scarce background knowledge of the context. Nevertheless, it is possible to retain the function that non-standard language had in the original text, by using a language that, for example, points to the poor education of a character (through the use of some form of low, perhaps grammarless language) thereby suggesting a specific and suitable social and cultural environment.
In Italian (my mother tongue) it is possible to suggest incorrect language by using (as in English) incorrect tenses. However, whereas in English typical non-standard forms would be a past participle instead of a past tense (eg. I done that yesterday) or perhaps a lack of agreement between subject and verb (eg. she don't or you was), in Italian the mistakes would be different (non-standard Italian could well be littered with the present simple or the conditional instead of the subjunctive, for example, giving Se tu eri più vecchio... instead of Se tu fossi più vecchio meaning If you were older...). In addition, colloquial expressions or abbreviations typical of the spoken language can be employed, such as 'sto instead of questo, which means this (such an abbreviation is a close equivalent of the English shortening of here to 'ere).
A good translation must be a balance between what gets lost and what can be found. The translation of non-standard language, even if it is a very difficult task at times, can be extremely fascinating and rewarding (at least in terms of personal satisfaction, since literary translators are rarely well paid). Once a skilled translator has become aware of all the implications of the original version, the next step is to muster up a suitable dose of creativity, to take try to find ways of making sure that as little as possible is lost and as much as possible is found in the translated version.
Italy