Some of the issues below are commonly recognized as unsolved problems; i.e., it is generally agreed that no solution is known. Others may be described as controversies; i.e., although there is no common agreement about the answer, there are established schools of thought that believe they have a correct answer.
Is it possible to formally delineate languages from each other? That is to say, is it possible to use linguistic (rather than social) criteria to draw a clear boundary between two closely related languages with a dialect continuum between their respective standard forms (e.g. Occitan and Catalan)?
Language isolates have no demonstrated relatives, and essentially form language families on their own. Can any of the approximately 159 language isolates be shown to be related to other languages?
Can we use the comparative method to reconstruct back to an arbitrary time depth, or do we need new methods to reconstruct the distant past of languages? Is there a time depth beyond which we cannot reconstruct?
Can we ever demonstrate that all languages are ultimately related to each other, or that they aren't?
Controversy: infant language acquisition/first-language acquisition. How are infants able to learn language? One line of debate is between two points of view: that of psychological nativism, i.e., the language ability is somehow "hardwired" in the human brain, and usage based theories of language, according to which language emerges through to brain's interaction with environment and activated by general dispositions for social interaction and communication, abstract symbolic thought and pattern recognition and inference.
Is the human ability to use syntax based on innate mental structures or is syntactic speech the function of intelligence and interaction with other humans? The question is closely related to those of language emergence and acquisition.
Is there a language acquisition device: How localized is language in the brain? Is there a particular area in the brain responsible for the development of language abilities or is it only partially localized?
What role does linguistic intuition play, how is it formed and how does it function? Is it closely linked to exposure to a unique set of different experiences and their contexts throughout one's personal life?
Linguistic relativity: What are the relations between grammatical patterns and cognitive habits of speakers of different languages? Does language use train or habituate speakers to certain cognitive habits that differ between speakers of different languages? Are effects of linguistic relativity caused by grammar structures or by cultural differences that underlie differences in language use.
Is perfect computational word-sense disambiguation attainable by using software? If yes, how and why? If no, why? (This presupposes the solution to the unsolved problems in the other areas of linguistics as a basis.)
To what extent are termbases reliable and can their reliability be measured objectively? If yes, how and why? If no, why? What is the relationship between termbases and individual subjectivity and can subjectivity about word sense disambiguation be overcome at all or is it a natural result of different experiences in one's unique personal life?