There are over 6700 distinct languages in the world today. Language surveys can be designed to ascertain linguistic relationships, degree of intelligibility between language varieties, language attitudes, degree of bilingualism, etc. Language survey is still needed throughout the world to help determine where Scripture translation is still needed. In a real sense, each speaker of a language may be said to have his or her own variety of the language. Pronunciations change from generation to generation.
Words shift in meaning.
The study of how languages change through time is called historical linguistics. They can study changes in pronunciation, phonology, morphology, syntax, etc. If there are no written records, linguists can still make educated guesses on what changes have taken place. Proto IndoEuropean Germanic Old English 450-1100 ca. Consonant clusters also tend to assimilate to the same voicing pattern.