Hiya!
I’m not familiar with the Korean language, so this answer will be a bit general.
With torchtext, you’re able to pass your own tokenizer to it. In a simple but extensible example check this out.
def tokenizer(text):
    """
    Function to tokenize given string of text.
    # Arguments:
        text: (str) String to be tokenized.
    # Returns:
        List of tokens.
   """
   return str.split() # This returns the list of tokens.

You can then pass it into torchtext using the following. TEXT = data.Field(tokenize=tokenizer)
To extend it to Korean, you’ll need to find or develop a tokenizer function that works for Korean 
Hopefully that helps!