Korean revision & minor correction for ja,en&zh_tw

[ianychoi]

Please approve this pull request.

Main revisions are : I smoothly revised Korean translation and added rest_qos chapter by seeing ja version book. : I found 'Faild' typo for all the editions, so I corrected. : I added 'translation preface' for Korean readers.

Thank you in advance!

[fujita]

Thanks!