A language survey is conducted around the world for a variety of reasons.
measuring people's ability to speak and understand another language
studying people's attitudes about different languages
evaluating the differences and similarities in speech of communities that speak related speech forms, noting comprehension or collecting details of linguistic form
assessing the vitality of languages that may be disappearing
doing initial descriptions of languages in areas that are linguistically undescribed
Methods
Methods used in language surveys depend on the questions that the survey is trying to answer. Methods used include collecting word lists, playing recorded texts to assess comprehension, sentence repetition tests, questionnaires, group and individual interviews, retelling of stories, direct observation, pointing to pictures after listening to instructions, and even internet surveys. As with any form of research, the methods used depend on the questions that the researchers are trying to answer. Also, the reliability of the results varies according to the method and the rigor with which it is applied, proper sampling technique, etc.
Applications
The results of language surveys are use for a variety of purposes. One of the most common is in making decisions for implementing educational programs. The results have also been used for making decision for language development work. And of course, academics are always interested in the results of any language survey.
Agencies
Surveys have also been conducted by ethnic associations, government agencies, NGO's, foundations, etc. Often such groups work together. Some large and notable surveys include the LanguageSurvey of India which was begun by George Abraham Grierson late in the 19th century and the Survey of Language Use and Language Teaching in East Africa, sponsored by the Ford Foundation from the 1960s. Both resulted in a number of volumes describing locations of languages, patterns of multilingualism, language classification, and also included descriptions of languages, such as Language in Ethiopia. The single agency conducting the most language surveys around the world is SIL International. The results of many of their surveys are posted on the web: http://www.sil.org/silesr.
Sign languages
Surveys have usually been conducted among spoken languages. However, surveys have also been done among users of sign languages. As with surveys among spoken languages, surveys among sign languages have studied multilingualism, attitudes about various languages both spoken and signed, differences and similarities between signed varieties, and assessing the vitality of signed languages, and initial descriptions of undocumented sign languages.
