If they are all Koreans, and they know English but not with great fluency, and they are on the job doing work for which English is not required, but getting the job done right is, it makes sense to use the language that everyone in the office understands with greatest subtlety, speed and accuracy.

Also: perhaps they resent being told how to think, which is what a language does.

If it's all about becoming more proficient in English, Major Lee can stipulate that lunches be all-English, and let the workplace alone.

Think about this, Justin: If you were pretty good in another language, and all the other people in your unit were also pretty good in it, would you all be doing your best work trying to stumble through your jobs in the limited vocabulary that you have in the second language? What would be the likelihood of errors due to incomplete or misunderstood communication?

Knowledge is supposed to be a key. It's being used here as a blunt instrument.