This phenomenom is known as “Language attrition” and is studied by linguists and psychologists.
The younger the age of transplantation to a new language environment, the deeper the loss of the native language, it seems. For example, I have documented that Korean children adopted in France when they were between 3-10 years of age do not even identify Korean utterances: See the papers at
http://www.pallier.org/papers/Pallier_cerebralcortex2003.pdf and http://www.pallier.org/papers/…
In adults, there is not always a big loss but word finding difficulties seem common.
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