I'm going to Busan in a few weeks to get my palm tree fix and be even more confused than usual because Busan Korean and Seoul Korean are kind like West Coast English and Highlands English. Technically it's the same language but we don't share words or pronunciations.
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I'm told my Korean has a very good Seoul accent. After two years of living here I can pronounce most the common words correctly and I know enough slang to understand the ads.
My English is becoming more Expat/Airport English and slang has almost dropped completely out of my daily vocabulary because my neighbors know Book English, not Daily Use English. I sound more formal in English now.