I’ve been fascinated by adults in Greece who can speak both Greek and English to native levels yet as they get older they stop being able to do both. This has perplexed me in the UK too with people who learnt languages fluently at a young age yet can no longer speak the language.
This isn’t just related to an average person though. If you look at Ricky Martin’s career he speaks with his native accent in Spanish and English but then he moved to the US and gets married to an American. His accent changes so drastically that he can’t even sing his own back catalogue properly anymore!
Luckily this change can be reversed as on his most recent song he is back to sounding how he used to when he was younger.
As the article above suggests it’s all to do with emotions. If you associate positive emotions with a language and a specific purpose then you are more likely to remember it. If however you try to use a language for another purpose like your native language for work purposes in an English speaking country your likely to fail. Too much mental control is required for this to occur.