I18n

If you use i18n, you have to include language files.

In app/assets/javascripts/application.js, you should add the following:

// load all locales
//= require summernote/lang

// or

// load specific locale (ko-KR)
//= require summernote/lang/summernote-ko-KR

Then, update Summernote's initialization options:

$(function () {
    'use strict';
    $('[data-provider="summernote"]').each(function () {
        $(this).summernote({
          lang: 'ko-KR',
          height: 300
        });
});