Someone already addressed this but I’d like to add to it.  RM specifically writes in both based on what sounds good, he’s said this in an interview.  If it sounds better all in one language, he’ll do that.  But if it sounds better to mix, he’ll do that.  And I suspect the other rappers do the same.

RM is fluent in 3 languages, and he uses this to his advantage when composing lyrics.

There are many reasons to mix languages in a verse. The Korean words might not have the right number of syllables to maintain flow.   This is the most common.

But RM has other reasons.  In many of his songs (not this one) he will use multi-lingual wordplay.

Example:  the word “일” is pronounce “ill” just like the word in English that means “sick”.  

So here is where the wordplay gets 4th dimensional. Sick can mean you have a cold, or it can be a compliment about how cool you are.

If RM’s verse begins “I’m ill” you don’t know yet which meaning he is using.  But in the song “Dis-ease” he switches to Korean and refers to the various things “일” means in that language as well!

“일“ means the number one, but it is also the word for “work”.  

He switches the meaning between all 4 definitions throughout the verse.  

Another bilingual play on words is in “Cypher 4” at the beginning.  He repeats the phrase “Sorry, Bae” over and over sarcastically. This word in English means something like “honey” or “sweetie”.  

But in Korean, it’s also the second syllable in the words 선배 and 후배.  
Sunbae (senior) and hoobae (junior) are polite forms of address you would use with people at school or work, or with colleagues in your field.  So if RM is addressing other rappers with more experience he would call them sunbae.  

All the grievances in the song are actually directed at his senior rappers, so this is where the wordplay comes in.

If he does mean “sunbae” he’s being rude calling them just bae. He’s being even more rude if he means “babe”.  And he’s being rudest of all if he’s actually implying that they are now the juniors (hoobae) to him. It’s a brilliant two language diss track.

The drummer in question being The anderson paak is so iconic

Yes,BTS members are really smart!
Intellectually, emotionally and musically wise !
That's why being an ARMY is such a bliss !😊

29:33 I’ve heard it described as RM raps from his mind, J-Hope raps from his heart, and Suga raps from his soul.

I love how they are so well trained and practiced that they can sit with their legs crossed and sing out live.

Born Singer (first song) is a remake of J. Cole's Born Sinner - the rappers poured out their feelings about their debut and the difficulties they went through.  They performed it live a couple times, then years later got permission to release it on an album and it is the first song on their Anthology album "Proof". It is a very special song to the group and ARMY!

6:25 The mixing of English / other languages with the main language (Korean in this case) is deliberate — phonetically and lyrically speaking. BTS has certainly mastered the art of it in their songwriting, but this is also a very common practice in Asian music (Japanese, Mandarin, Philippino etc.)

J-hope raps like he dances....bouncy

Anderson Paak is the other half of Silk Sonic with Bruno Mars. His mother is Korean as well as his wife so he speaks a little Korean.

NEVER...NEVER will there be a group of true musicians like them...NEVER...they are excellent singers, composers, producers and not to mention their VALUES AND PRINCIPLES....THEY ARE UNIQUE.....

Jk harmonizing and adlibs so pleasing to hear..

The drummer that was living his best life is Anderson Paak, an American singer, rapper, songwriter, and record producer. He gained recognition for co-writing and singing on almost half the tracks on Dr. Dre's Compton. Known for his collaborations, he's worked with Korean and Korean-American artists like DEAN, Dumbfounded,Tokimonsta and BTS.

To me the rapline has very distinct styles.
RM is the poet, he has complex, emotionally charged lyrics
Suga is the genius producer, his music is the vehicle of his emotions
Jhope brings texture, versatility, tempo and rhythm changes.

The three of them are insanely good at what they do and, as a very biased listener, I believe they are the most talented and balanced rapline in Kpop.
I love them very much 💜💜💜

J-Hope's verse as you note in "For Youth", has a different esthetic from RM and Suga. For me it  clearly shows his roots in rhythm and actual singing/acting - he actually has a great "musical theatre" full-bodied voice and sings with more sensitivity and nuance based on dramatic interpretation. All three of them are singing their verses on set melody notes in this song, not rapping with a speaking voice.
Suga often uses an auto-tune or robot effect when he sings; I believe mostly it is a purposeful style or color choice; his long experience as a producer gives him a broad sound spectrum to play with.  He pays careful attention to pitching raps appropriately for the song and raps his carefully crafted verses with urgency and unfiltered, often  wrenching emotion, from the heart and the gut.
RM has deep feelings and complex thoughts that shoot like lasers out of his heart and mind - so much to say and so little time! But he is also so loved for his softer, introspective and soulful singing.

I would say their ability to say the words in the way the rappers do, is how Suga would describe it as tongue technology.

Their melodic rapping is what makes their music so special.  They give us everything. So much talent. RM is a master lyricist. He makes smart music. They all do.
I cant wait to have them back!!!!

Jhope actually learneddd how to rap when he joined the company as a trainee, he learned by Rm and suga’s side and created his own style, while also giving some nice vocals in bts’s songs, and ofc being the main dancer he is def an all rounder

I'm borrowing from a comment I made to another reactor who called out the uniqueness of j-hope's rap, to explain what I think sets him apart: "j-hope is first, in his deepest heart, a dancer - a street dancer since he was a young teenager. For that reason, you are going to hear some very different music from him - because he writes music as a dancer, not a musician. So he'll take a beat, and rather than "riding" it like you would expect, he does really interesting things with it.  Also, he is known for his ad libs within BTS, so he also does a lot of that in his music. j-hope once said (I'm paraphrasing, because I don't remember the exact quote) that he makes music from the rhythm of his body. When he writes or hears a beat for the first time, he often envisions the choreography he would do with it. All of this makes him a really unique artist. I love his music for that reason." So, I think it's true that his rap is emotionally driven, because I think dancers are trained from the beginning of their careers to be able to express emotions. j-hope has taken that and applied it to his rap style, and it really makes him unique. It's how he differentiates himself from the two rapline members who started out rapping (and from whom he learned to rap).

RM is an amazing song writer, since he's good at english, he's able to switch between both languages very easily.