Two anecdotes related to topic:
1) The benefit of having been raised bilingually
My best friend at the University of Georgia around 1990 was a Mauritanian/French national - partents both born in Mauritania, he grew up mostly in France, with both Arabic and French as first languages. Ahmed was the most language-talented person I have so far met. A few years later, I visited him in Spain for two weeks, a few months after he had moved there, to take a job in Madrid. He had gone there with no previous Spanish lessons, but by the time I got there, Spaniards would ask him what region of Spain he came from (his accent, apparently, never sounded fully local, but good enough to plausibly be taken as a dialect; his grammar apparently was flawless, vocabulary extensive). Meanwhile, American tourists asked me what country I was from, and asked him what US state he was from, because the American English he had acquired in Georgia was convincing.
Whenever Ahmed got in brief contact with a language, he'd pick up usable bits in no time at all. His German was good enough to chat with my mother on the phone who spoke no foreign language whatsoever, and he even could speak Romanian and Greek.
I have little doubt that his talent is related to his bilingual infancy.
2) I have a list with the phrase "I can't speak [name of the language]" in almost 40 languages, and have a majority by heart. Most European languages (all of the Indo-Germanic family) are rather easy to remember and fairly easy to pronounce. Finnish, Estonian and Hungarian, though not related to the Germanic or Slavic languages, are easy, too (just remembering and speaking a single sentence!). The Indoarian languages of India pose no great challenge, while the Dravidian are terrible. Japanese was one of my first list entries, and while I am unsure I get the tonality correct, I can remember it well and what I say I say fluently. I struggle however with Mandarin and Arabic, and the
worst was Korean: While in Spain with Ahmed, we took a night train from Granada to Madrid and shared the compartment with three young Korean women, college students claiming to study Spanish, although Ahmed assured me that their Spanish was even more lacking than their bad English. Anyway, I managed to explain to the girl next to me that I want to learn to say "
I can't speak Korean" in Korean; if she could write that down. Sure, she said, and wrote it down - in Korean script. No no, I'd need that in Latin characters. That was a difficult thing to ask from her! She wrote something down ... I read it out load, and she laughed, I spoke it entirely wrong. So I asked her to pronounce it to me - over and over and over again! I tried to write down phonetically what I thought I heard, but (with train noises around), I was struggling hard to identify phonemes. We spent certainly more than one hour trying to get the pronunciation across - and yet I failed. (The only upshot to this drawn-out failure was that she was young and pretty and had her lips close to my ears for a substantial portion of the ordeal...

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