Aww, thank you! I do my best.
That said, I will always second myself on the finer points of language, but I had that sense about Rengoku-san too. I love the "umu!!" XD Another character who speaks in a very similar manner is Kija from Akatsuki no Yona, whose village maintains very archaic customs, so the old speaking style kind of makes sense.
I went to a native speaker and fellow nerd friend to get her take on this (since you mentioned knowing a little Japanese I'll leave her comments as-is):
Rengoku-san often use words we don't use often nowadays.
Like, "うむ!" or"よもやよもや" or "穴があったら入りたい!".
Some of my friends and I even searched the meaning of よもや on Google lol!
In Kimetsu, Gotoge-sensei (often called as ワニ先生) use different way of speaking according to how the characters grew up.
Since Rengoku-san seems to have a rich family with an historical background, he speaks like well-bred.
It doesn't sound like Samurai but it sounds dignified.
On the other hand, Sanemi speaks more roughly.
He had a bad father and his family looked not rich.
ワニ先生 said he went around poor and insecurity area to kill demons on his own before he enter Kisatsu-tai, and the way he speaks got rougher than when he was living with family.
In Japanese version, he stretchs out the end of the word. ex)何だァ?
Other characters also have some insignificant habits in the way they speak but the comparison of Rengoku and Sanemi is obvious.
But in the translated version, I think those expressions have been losted.
It's Taisho period but not too far from Edo and people still had older way of life especially in countryside like Tanjiro.
Teoni (the demon who killed Sabito) said Urokodaki-san trapped him there 47 years ago in Keio era which is a part of Edo period.
When you calculate the years, you can see the story sets somewhere between Taisho 1st to 4th.
It means many older people were born in Edo period, and people still had old way of speaking.
I hope it helps😉
(She is indeed so helpful. And a licensed tour guide if anyone ever needs a recommendation for one. : D)
I went on to tell her about how Genya's switch from Aniki to Niichan was translated as going from Sanemi to Nemi. She and a literature translator friend and I all agree, in working with a purely English translation, that was a cute and clever move. : 3
I have not seen Mugen Ressha with subtitles, I do not know how distracted I would be by translations of his speech manner. xD If I were to try to capture the same effect and really have fun with it, I might have him say things like "Preposterous!" or other slightly stiff but very confidently used phrasing of things. But in looking up a list of synonyms for "yes" to find something that seems right for "umu" (I was thinking "affirmative" but that seems to big a mouthful and too cold), I saw "amen" and now it's past midnight and I am laughing to myself at Rengoku-san shouting "Amen!" as a catchphrase. XDDD Wouldn't exactly make for a non-distracting translation.