What Nionoi is saying is that you can buy a retail license - and any retail license that doesn't explicitly say "Single Language" will let you use multiple language.
I have typically seen that SL editions of Windows come preinstalled from OEMs, while retail licenses are not SL. You can't change the language of the SL edition that you already have, it must be replaced with a retail-type license if you want to change the language.
I can't read Korean, but if you go to the S. Korean Microsoft Store website, it seems to list Windows 10 Pro for purchase: https://www.microsoft.com/ko-kr/store/b ... areWindows
EDIT: There is a question on Stack Overflow that has an answer detailing how to change the display language for a single language edition. I'm not posting the link here, as it is quite possibly a violation of the Windows license terms (and by posting that link I'd be afoul of TR's forum policies). Moreover, such hackery will quite likely result in an unsupported configuration that might work on day 1 but breaks down the line (e.g. the next half-yearly update of Windows 10 might break). Nionoi's advice is the best, pick up a retail license.