I am very good friends with a Finn who has only been to the U.S. for one week of his whole life. In normal English conversation, you would not know that he isn’t American. The only time he has an accent is when he uses a multi-syllable word that he rarely uses.
He was taught English in school from ages 9-18, but learned how to speak from watching American movies! It’s quite impressive.
I was at a Barcelona hotel waiting in line to talk to the clerk at the desk. I was third in line. I listened to him talk and thought that he was an American and had found a great summer job in Spain. When I got up to the front of the line I asked him where he was from, thinking he would say Cleveland or Nebraska or something. He said he was from Barcelona. I complemented him on his command of the English language. He said he had a teacher in grade school who was from Pittsburgh who taught him correctly. I was really flabbergasted. He spoke better than most big American city high schoolers.