I wish I had language notes for Saiyuki and Saiyuki Reload, but I don't have copies of the original manga or raws.
So, completely random things I noticed:
Formality Levels
Goku, Konzen and Kenren all term themselves as "ore" and use "omae" as the second-person pronoun. This wasn't a surprise for Konzen and Kenren, but it does sound a little funny having wee 8-year-old looking Goku using "ore." Ditto with Nataku. Ditto with Kanzeon, which totally cracks me up.
Tenpou uses "boku" and "anata." I don't know if Hakkai does or not, but I wouldn't be surprised at all.
Kenren and Konzen and Goku all use very informal Japanese, Kenren in particular. He changes all the "~nai" sounds to "~nee," which I think is rougher and more casual. Konzen does a little as well. Minekura also indicates this visually by occasionally writing out the longer vowels with dashes. Ex. "Sou jya neeeeee yo" (That's not so) becomes "So~~~~~ jya ne~~~~~ yo!" For more emphasis, occasionally particles are written out in katakana instead of hiragana. This is a pretty common thing in manga, not just a Minekura thing.
Tenpou ends everything properly and with formal conjugations, instead of using casual form. Ex. he conjugates the verb "iku" (to go") as "ikimasu yo!" (Let's go!) instead of saying something like "iku zo!" (more masculine)
Actually, everyone is fairly informal except for Tenpou, which I think is rather funny. It makes sense in terms of their Saiyuki reincarnations, except in this one, Konzen is the non-militant, albeit extremely grumpy, one.
I absolutely adore the language in the first meeting between Tenpou and Kenren in vol. 2. Kenren's very polite at first when he knocks on the door, but when no one answers, he sort of barges in, only to be barraged by Tenpou's Pile o' Junk. The first snatches of conversation with Tenpou are still fairly polite; I think Kenren thinks he's an aide or something. Kenren asks very nicely if Tenpou Gensui is around, and Tenpou answers that it's him.
Kenren stares, bug-eyed, and then laughs, saying very casually, "Oh man, you almost had me there!" (or words to that effect), at which point Tenpou laughs and says, "Me too!"
They stare again. Kenren realizes Tenpou isn't kidding and quickly kneels and introduces himself in incredibly polite Japanese and asks Tenpou to forgive him. It's all full of "moushiwake arimasen" and "sashi agemasu" and whatnot, which are phrases that you definitely use with a superior.
And Tenpou continues on normally and can't find his hankou, and Kenren starts talking like Kenren again.
Hrm. Doesn't sound quite so fun here, but I was so totally amused by Kenren being ultra, super polite.
Names and Kanji
Konzen Douji - "Kon" means "gold," "zen" means "cicada," "douji" means "child," though I suspect it could very well be some archaic Chinese rank or something.
rachelmanija remarked that the gold fits because of Konzen's hair and the sun/light imagery consistently associated with him. I, of course, am amused by the cicada bit, though I have no idea what that's supposed to signify. Summer? The sound of cicadas are always used to signify summer, and that would go with the sun thing. Also, I snerk because the "zen" for cicada is very close to the "zen" for Zen Buddhism, except the "god" radical has been changed to a "bug" radical.
Kenren - not going into Taishou, as that's just a rank. "Ken" means "to curl or roll" and "ren" is "curtains" or something like that. I have no speculation, except the "ken" just seems to fit because the radical is a hand and it feels very action-y. Of course, I may just be saying this...
Tenpou - "Ten" means "heaven or sky," it's the same ten as in tenkai (Heaven/World above/etc.), "pou" is a kind of plant that for some reason I associate with a waterlily. I think it's because you can say "lien pong," the first lien being lily and pong being pou being... some additional plant thing stuck on to make it a phrase. Again, I liked this for Tenpou just because it feels more in the air, more removed from the physical.
And then there's Goku, whom Konzen names. I think he originally says he'd just name Goku "Saru" (monkey), but when Goku protests, he decides on "Goku" (technically Gokuu) because it's a short name that even a monkey brain can hang on to. I think he says the "go" is the kanji for "satoru," or "to realize" and the "kuu" means "space, or something to be filled." Alas, I cannot look up the exact thing Konzen says =(.
ETA: Ok, found it. It's not really Konzen saying anything, just sort of free-floating text reading: "Kuu" -- "something that can't be seen with the eyes. Something that can reach 'enlightenment' (satoru, the verb formed from the kanji for "go")." As
rachelmanija says below, very Zen, which isn't surprising, given that it's the name given to Monkey by the guy who teaches him to change shapes and surf on clouds. In the book, "go" was the set generational given name beginning kanji for the teacher's students, and I think the teacher picked "empty" ("kuu") because of its very Buddhist connotations. Also, the "go" for "Goku" is the same as the one for "Gojyo" and "Gonou."
Everyone also has ostensibly Chinese names; Konzen, Kenren and Tenpou don't have surnames, but their given names consist of two kanji. Li Touten has a pretty Chinese-looking name; "Li" is just a common surname and I sadly can't remember what "Touten" is.
Assorted Notes
I have no idea how the translations refer to Heaven and Earth; in the manga, Heaven is "tenkai" and Earth is "gekai." The "kai" in both refers to world. The "ten" for Heaven is fairly normal; it usually means "heaven" or "sky." I don't think "tenkai" is the standard term for Heaven in the religious sense, but it definitely works here, given that Heaven is a world in itself and functions as such. I like "gekai" just because "ge" means "down or below;" it's literally the world below. So I keep futzing up because I'm not sure if I should just call it Earth, or Down Below, or what. I also keep associating it with hell because of the down bit, but that may just be me.
ETA2: more translation notes below in comments