It is not recognized as literary language. The rules governing it are not standardized.
The common use of dialect cannot be found in dictionaries so it is not recognized as a literary language. It also does not have any standardized rules as a dialect is more often heard rather than written. Common dialect is through the way words are pronounced and/or put together. Much of Twain's writing is written in a dialect that is common for characters of that setting. However, another author may use a similar dialect but have different spellings or punctuation choices for the same sentence.