There are many opinions about the usage of Korean in fanfics. Some will tell you not to use it, others won't care. Since there are a lot of stories here about Korea and we are all in love with Korea, I think it's only natural that some writers use occasional Korean words. Most of us know a lot of basic words, anyway. It's also true that if you want your story to seem more professional and mature, you will have to give up on using Korean. Personally, I think that excess use of Korean words is a turnoff, especially if they pop up randomly and break the flow. Not everyone is familiar with Korean language either.

I used to add a lot of Korean in my stories but sort of gave up on that long ago and now only use the most basic words that determine status like hyung, ahjumma, oppa etc. If you want to make your story seem more Korean then it's a better option to do it through people's behavior, beliefes, the way they think etc. Culture is a lot more than just a language!

My views:

1. Minimize the usage.

2. If you must then use only simple words.

3. Use Korean only in dialogs.

4. Provide translations.  ("Anyo! No!" I shook my head. / "Saranghae. I love you", he whispered. or a list words&translatons at the beginning/end of the chapter.)

There are some words that can't be literally translated like: oppa, unni, noona, ahjumma, ahjussi etc Sometimes the word loses it's meaning or its power when it's translated and sometimes it's hard to translate the formalities like hyung - hyungnim (banmal jokes are totally impossible to fully translate). Let's take a look at some basic words and their replacements.

Aish - Damn /  Aishshi! - God Damn it (not exactly but close enough)

Ang - Awww (totally different implementations in real life)

Gaesekki - Son of a dog (a bit weird) --> , .

Imma - Hey you/ you there/ you (however the impolite/rude shade of the word fades) 

Yah! - Hey! (let's face it, yah is much cooler)

Ahjumma/ahjussi - old man/old woman (is much ruder in English than Korean)

Babo - Stupid (equal)

Shibbal - you (equal)

Michin/Michigesso - crazy or you mad?/ I'm going crazy 

Michinnom - Crazy bastard  (cruder in English)

Gabjaki - Suddenly!/So suddenly!/ What a startle! --> more like Oh my God you scared/startled me!

Sajangnim/ sajang - teacher/ teach --> replace with the Mr./Ms. + the name of the teacher

Gyosunim - professor (neutral)

Wae - Why (neutral) 

Saranghae - I love you (equal)

Kwioptta - cute (equal)

Omma/appa - mom/dad or ominim/abunim - mother/father (equal)

hyung - older male between males with a close relationship (or family relationship) --> brother --> bro, man, buddy, my man (doesn't point out the age difference)

hyungnim - older male + respect --> Boss, Sir. or just use the name of the person. 

Unni - same with hyung but between females --> sis, my girl

Agassi - young miss (equal)

Yobo - honey (equal)

Omo/Omona - Oh my (equal)

I think, the hardest part is to translate the feeling of the word and the cultural weight. We use words differently in a different language, that's obvious. Anyway, what I wanted to point out is that maybe sometimes going for the exact translation is not the best option. Stop and think, what would be the best word in the situation you are describing? Also, those words that can be translated with equal weight and meaning, I don't really see why use the Korean version. 

Sorry about this super short post. It was something I wanted to talk about, but in the end there was nothing much to say. LoL. I just hope this is helpful to someone out there. Sorry, to anyone who was left feeling disappointed with this post. I know what you are thinking: