Fix Korean translation (#4065)

* Added Korean Translation (based on japanese)

* Update korean translation

* Update korean translation: fix syntax error

* Updated korean translation

* Update korean translation

* Update ko.json

Translate non-translated parts

* Update ko.yml

Translated missed parts - and fixed some typos

* Create simple_form.ko.yml

* Update simple_form.ko.yml

Translation error fix - password change form

* Update simple_form.ko.yml

* Update ko.json

Missing translation