just to preface im not a native jpn speaker nor do i know nc and jju personally so literally take these as the ramblings of a crazy person. these are all simply my thoughts, opinions, and assumptions from what theyve given us themselves as well as from someone who spends a lot of time listening to them speak and as someone thats also learning the language just like them

anyway moving on…i think you can chalk up the way they speak to their roles and just how they are as people. ej definitely talks more than nc when it comes to broadcasts/radio/interviews partly because nc likes to be a listener and partly because ej is the leader. i do think ej being a leader has lead to him speaking, i guess you could say, more seamlessly than nc does because he has no choice but to. he is the bridge between company/staff and group and there are probably plenty of situations where he has to talk on behalf of the team as well as on behalf of the staff to the group (you know the whole 7 jpn members thing) so getting the point across succinctly and in an organised manner is sort of a skill he has refined over the years
i guess you can also say the whole “the more you use a language the better you get” thing could play into it too. ej only has to switch between jpn and krn on the daily. if he is not speaking in jpn then he is speaking in krn so there probably is something to be said about him spending more time recognising and ironing out his personal kinks when it comes the language. then you have nc who actively switches between four different languages like his mind is definitely just a stew of it all so honestly the effort he put into being as fluent as he is in his fourth language is so commendable (like come on he was directing his entire wenoism shoot in this fourth language of all things)

i will also say that although they obviously both rely on a hands on approach when it comes to language, nc is especially more so in the way that ej does seek other modes of learning/input (actively learning kanji, tries to read jpn books, listens to more jpn music) which i think is why ej does seem to be able to actively pull from a wider vocabulary pool when speaking. he also has beautiful word choice and expresses his thoughts in quite moving ways. nc has always had a “learn by speaking and absorbing from environment”approach and is a big advocate of talking by 感じ (also i just think he has a talent for language) so hes a fluent conversationalist but he does end up pondering (cue filler words) more with topics he doesnt talk about as often (obvi same with ej but again more seamlessly put…also nc easily full confidence on regular topics or topics of extreme interest). and will also say its also fun that nc reads easier (despite hating it) since knowing mandarin helps him to read kanji and deduce meaning more intuitively

for observations i feel like they both use filler in similar amounts (yeah maybe a tad more on nc’s side) but i think it feels less prominent with ej because he tends to recover his thoughts quite quickly, gives himself the time to recollect them internally, as well as he mostly uses pauses and stops himself before he continues on with a rewording so he can say what he wants to say (i find he typically uses あの/なんか/えー/こう) whereas nc tends to get caught in the trap of trying to explain himself, changes thought/wording multiple times mid sentence, and he also recollects his thoughts out loud (i find he typically uses あの/なんか/なんだっけ/なんという…sometimes もう etc)

some other habits would include nc’s そうですね before he starts answering questions on broadcasts and especially radio shows (i can no longer unnotice this…). nc also pronounces some words in a nc way but he does it consistently so you kind of just know what hes saying and recognise it as a nc quirk (like how he says きゅうに　 more like ぎゅうに). nc also pronounces some eng loan words the krn way rather than the jpn way (example i can recall most clearly is saying 데뷔 rather than デビュー)
as for ej im not too familiar with his quirks that arent really filler but he has a いやでも habit (does he have a 아니 근데 habit in krn?…i dont consciously pay attention to his krn word choices but itd be funny if it transferred over…). i will say when it comes to kanji nc does the funny bit of just saying unknown ones in mandarin until someone corrects him whereas ej seems to know the kunyomi/onyomi of singular characters which he uses to try and deduce pronunciation but then muddles the full word up with similar sounding words

also if you ever find yourself thinking nc is a tad harder to understand in jpn (he slurs and ej arguably doesnt he enunciates well…also nc occasionally doesnt finish his thoughts himself), ive seen thats a universal experience. recalling the time rua said sometimes kei misunderstands what nc is talking about and they end up looking like people having two different conversations. but also how rua, who is the no.1 nc understander, also frequently has his どういうこと check ins right after nc says something (but thats also why hes a no.1 nc understander…call it the literal “to be loved is to be known”)

…ill stop now since i got a bit carried away but i hope this answered your question in some way or another anon <3

YES i caught that too! theyre a specific trio that usually speaks krn when theyre together and fm has the tendency to default to it when promoting in kr so it was just interesting to me that fm was mostly talking in jpn while jju stuck with krn (although they were obv both mixing) but i was especially interested in how nc, who i feel tends to change over to krn like a switch especially if jju is talking to him first in krn, was purely replying in jpn (could be for a number of reasons: fm mostly leading the situation in jpn, heightened emotions = easily expressed in jpn since its arguably his most used verbal language, simply too put out to switch around etc)…anyway yeah…although wouldve preferred to see this language dynamic play out in any other situation that wasnt exploiting a visibly upset nc…