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An unclassified language is a language whose genetic affiliation has not been established, most often due to a lack of data. If a language's genetic relationship has still not been established after significant study of the language and efforts to relate it to other languages, as in the case of Basque, it is considered a language isolate; an unclassified language is therefore one which may belong to an established family once better data or research is available.
Languages can be unclassified for a variety of reasons, mostly due to a lack of reliable data but sometimes due to the confounding influence of language contact. Some poorly known extinct languages, such as Gutian or Cacán, are simply unclassifiable.
An example of a language that has caused multiple problems for classification is Mimi of Decorse in Chad. This language is only attested in a single word list collected ca. 1900. At first it was thought to be a Maban language, because of similarities to Maba, the first Maban language to be described. However, as other languages of the Maban family were described, it became clear that the similarities were solely with Maba itself, and the relationship was too distant for Mimi to be related specifically to Maba and not equally to the other Maban languages. The obvious similarities are therefore now thought to be due to borrowings from Maba, which is the socially dominant language in the area. When such loans are discounted, there is much less data to classify Mimi with, and what does remain is not particularly similar to any other language or language family. Mimi might therefore be a language isolate, or perhaps a member of some other family related to Maban in the proposed but as-yet undemonstrated Nilo-Saharan phylum. It would be easier to address the problem with better data, but no-one has been able to find speakers of the language again.
It also happens that a language may be unclassified within an established family. That is, it may be obvious that it is, say, a Malayo-Polynesian language, but not clear in which branch of Malayo-Polynesian it belongs. When a family consists of many similar languages with great degree of confusing contact, a large number of languages may be effectively unclassified. Families where this is a substantial problem include Malayo-Polynesian, Bantu, Pama–Nyungan, and Arawakan.
There are hundreds of unclassified languages, most of them extinct, although there are some, albeit relatively few, that are still spoken; in the following list, the extinct languages are labeled with a dagger (†).
Some 'languages' turn out to be fabricated, as Kukurá of Brazil.