Language identification is the process of determining which natural language given content is in. Traditionally, identification of written language - as practiced, for instance, in library science - has relied on manually identifying frequent words and letters known to be characteristic of particular languages. More recently, computational approaches have been applied to the problem, by viewing language identification as a kind of text categorization, a Natural Language Processing approach which relies on statistical methods.
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In the field of library science, language identification is important for categorizing materials. As librarians often have to categorize materials which are in languages they are not familiar with, they sometimes rely on tables of frequent words and distinctive letters or characters to help them identify languages. While identifying a single such word or character may not suffice to distinguish a language from another with a similar orthography, identifying several is often highly reliable.
This can be done by comparing the compressibility of the text to the compressibility of texts in the known languages. This approach is known as mutual information based distance measure [1]. The same techniques can also be used to empirically construct family trees of languages which closely correspond to the trees constructed using historical methods.
Another technique, as described by Dunning (1994) is to create a language n-gram model from a "training text" for each of the languages. Then, for any piece of text needing to be identified, a similar model is made, and that model is compared to each stored language model. The language model which is most similar to the model from the piece of text is the most likely language. This approach is problematic when the input text is in a language there is no model for. In this case, the method returns a random, "most similar" language as its result. Another problem are pieces of input text that are composed of several languages, as is common on the Web. For a more recent method, see Řehůřek and Kolkus (2009).