I read an interesting article in Psychology Today.
Part of the article stated the following: „ A second topic we spent some time on concerned whether you can lose a first language in adulthood. (In an earlier post I discussed this relative to children forgetting their native language; see here). Noam Chomsky didn't think you could and argued that there is some sort of residual storage. He took the example of a 60 year-old man who hasn't spoken German since age 20 and who no longer seems to be able to use it. The real test for him would be to see how quickly he could relearn it. He was convinced that the person would learn German a lot faster than if he were starting from scratch and he added that he would probably learn it with the right pronunciation, the right nuances and so on. As he stated, "My guess is that you can't really erase the system.“
I tend to agree with him. I heard a German film crew interviewing Henry Kissinger on YouTube once. He is a native German speaker, but he left Germany with his family when he was 15 (they may have spoken it at home, I have no idea) but he’s lived in the US since he was that age and is 94 now. When interviewed he was at least in his late 80s. He apparently rarely speaks German and seems not to want to. So, when he sat down for the interview, he asked the German film crew „Can we speak English?“ The interviewer said, „Of course, if you like, but I’d love to hear your German.“ He said, „Well, I’m going to sound like a 15 year old, be we can give it a try.“ He then began to speak and it was truly amazing. After 70 or more years living in the US speaking English, his German was about 99.99% perfect. Pronounciation, grammar, vocabulary, everything. I was shocked. Kissinger isn’t especially well-liked in the US or much around the world, and one of the German commentators on YouTube wrote, „Oh my God. This pig speaks German!“ Another person said, „once you learn it you never lose it.“
I think that once you have completed childhood, your native tongue has been anchored so deeply inside that you truly will not forget it. I left Germany two decades ago - and have been thinking, dreaming, and writing in English ever since. We hardly ever speak German at home (it is a 6th language for my wife), and when I am on a business trip back "to the old country" I find myself constantly searching for words during the first few days ... but: when I am really tired, or when the volume is really low, I find I still understand languages best in the following order:
German > English > Spanish > French > Chinese and so on
I have tested that "scientifically" by turning down the volume on movies in different languages to see how much I can still catch - and surprisingly, my native tongue is the most resilient! The others follow roughly in the order of length of exposure - the brain is a pattern recognition device, and the more you train it, the better it gets. But: even though by now the time I have spent speaking English is equivalent to the time I have spent speaking German, the latter still has some residual priority. Interesting, eh?
Children who are adopted at the age of four or five years by parents who live abroad are forced to forget their native language, give up their identity, their original name. I don’t know if they keep some kind of predisposition to learn again their first language. I guess so.
Kissinger wanted to forget German, to better integrate in the US as many migrants do. He had a very heavy German accent. Linguists even take his example to explain that children until a certain age can adopt easily a new accent but after the age of 15 it is difficult (but not impossible).
I like the points that Lee and Aurelio make because they are consistent and also coincide with the example I gave and Chomsky's opinion on the subject. The native language forms a sort of base language in the brain and new languages can be added, but it's not quite the same, even once you reach native-level fluency. I agree that you can never truly lose your native fluency, but sort of like Lee alluded to, and Aurelio as well, you can get a bit rusty by not using it. Yet, that fluency is regained rapidly with only a little effort. It's a bit like the spare tire in your car (assuming it's a real tire, not one of those little temporary ones). It's always there and always in good shape. It might lose a little air, or get a little dusty, but it never disappears and never stops being a tire, and likely never loses all its air, so it's always useable. Not a perfect analogy, but similar. I noticed the same issue myself Aurelio mentioned about his native German, only in reverse. When I lived and worked in Germany for an extended period, in Braunschweig (or just 10 km outside the city), I never spoke a word of English with the family I lived with (they didn't speak English anyway) and I never spoke English at work obviously. So, after a short while, I thought in German, dreamed in German, and never used English at all. I never even thought about it. But, then I wrote home once to my mother (no email in those days and no internet) and she wrote back asking why my English sounded so weird, the spelling was off, and the sentence structure seemed odd to her. Then when I returned, to the US, it didn't take long to readjust to English. But, she did ask again why my English seemed so lousy over in Germany. I didn't notice it, but I probably just got a little rusty with it. An interesting question would be, what would happen to a 5-year old American child, adopted and raised in Germany after that age? Would that child still be fluent in English? Interesting.