YOU SAID:

I spent most of my time studying Japanese language and Korean language and history

INTO JAPANESE

私はほとんどの時間を日本語と韓国語と歴史の勉強に費やしました

BACK INTO ENGLISH

I spent most of my time studying Japanese, Korean, and history.

INTO JAPANESE

私はほとんどの時間を日本語、韓国語、歴史の勉強に費やしました。

BACK INTO ENGLISH

I spent most of my time studying Japanese, Korean, and history.