How long would it take for you to 'forget' an unused language?
Essentially, no one knows: Excluding anecdotal evidence, there is very minimal research on long-term language disuse - partly because it is such a rare phenomenon, and difficult to measure. Nevertheless, I would estimate the time frame to last at least a good 20+ years, before seeing symptoms of ‘forgetting’. And ‘disuse’ constitutes not listening to, overhearing, reading, seeing, or speaking the language whatsoever.
Research has shown that children tend to forget their birth language when adopted at a young age (0–2 years) suggests that linguistic ability, even if unpractised for up to 20 years, can more easily be re-gained if learnt at an early period of life.
Further research has similarly shown that rather than going ‘dormant’, the phonology (sound patterns) of the unused language is impaired, rather than linking the idea to the language itself (i.e, the concepts involved).
There is further evidence that one can hold onto their childhood language after almost a decade of disuse.
Why does this occur, neuromechanistically? It has been hypothesized that the absence of non-linguistic phonemes results in the atrophy of the neural connections involved, consequently yielding more primitive neural circuitry (i.e, wiring) for those specific sounds. [The process of ‘reduced neural activity’ occurs through the neuroplastic mechanisms of Long-Term Depression].
Further theories suggest that for language acquired at an early age, the brain organizes itself dramatically (through neurogenesis and synaptic pruning), following which a change in input will result in delayed reorganization. Due to this tardy reorganization, the neural connections characterizing native linguistic ability remain for a very long time.
Moreover, language has shown to be processed in several areas of the brain, including the temporal cortex, the inferior frontal cortex, and corpus callosum. Crucial regions involved also include Broca’s area (largely responsible for speech production) and Wernicke’s area (contributes greatly for speech comprehension), and the angular gyrus. The structure-specific processing of language through such a diverse network likely contributes to keeping it alive, despite years of misuse.
Most of these regions are visible here: