Yes, forgetting is forgetting, has been the belief ... you 'use it or lose it' ... but language learning may instead be more like 'riding a bike' and even a "forgotten" language may be more deeply engraved in our minds than we realize.
Psychologists Jeffrey Bowers, Sven L. Mattys, and Suzanne Gage from the University of Bristol recruited volunteers who were native English speakers but who had learned either Hindi or Zulu as children when living abroad. The researchers focused on Hindi and Zulu because these languages contain certain phonemes that are difficult for native English speakers to recognize. A phoneme is the smallest sound in a language—a group of phonemes forms a word.
The scientists asked the volunteers to complete a background vocabulary test to see if they remembered any words from the neglected language. They then trained the participants to distinguish between pairs of phonemes that started Hindi or Zulu words.
As it turned out, even though the volunteers showed no memory of the second language in the vocabulary test, they were able to quickly relearn and correctly identify phonemes that were spoken in the neglected language.
The findings in Psychological Science suggest that exposing young children to foreign languages, even if they do not continue to speak them, can have a lasting impact on speech perception. The authors conclude, "Even if the language is forgotten (or feels this way) after many years of disuse, leftover traces of the early exposure can manifest themselves as an improved ability to relearn the language."