Add Korean to the list of supported language analyzers: https://docs.atlas.mongodb.com/reference/full-text-search/analyzers/language/#ref-language-analyzers

CJK Analyzer is not fully support the Korean one so there might be specific Korean analyzer out there.

For example 'Nori' which is Offlicial Elasticsearch Pluging For Korean Language Analysis. 'Nori' which is based on mecab-ko-dic.