Mermaid Swamp Notes
Not a lot of translation stuff to explain, but here you go. Spoilers for the whole game, as I also clear up some things about the plot.
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What in the Fresh Hell
Rin, Seitaro, and to a lesser extent Yuuta all talk pretty roughly, so yeah. College students, am I right? ("College Student" wouldn't fit on the menu, so I put the "college" part in my short summary instead.)
Seitaro likes to call Yamazaki "yamazaru," which means "bumpkin." Sometimes he just shortens it to "kuso zaru," "damn monkey." I translated this in various ways, sometimes depending on the context, like "Yammerzaki" when he's telling her to shut up.
Of course, sometimes I just had him say "Yamazaki," because it sounds exasperated enough as it is, and I figure he can't be bothered to think of a play on it every time.
Feely-who?
There's not much to say about Rin's illiteracy; the stuff she can't read is fairly antiquated with a few obscure kanji, but I'd still say it's pretty laughable that she can't even guess. But 119 is Japan's version of 911, so no, that at least isn't her being dumb.
If you were too lazy to look it up, Yaobikuni (literally "800-year priestess") is the legendary nun the old man talks about who ate the flesh of a mermaid and lived 800 years.
Uri mentioned in reply to my translation that the game must have been hard to translate because of "lots of antiquated expressions," and gave Yaobikuni as an example, but, uh... Really, it's just those two things with old Japanese and the name of a legend which was better left untouched. Not any especial difficulty there.
Orochi
I completely forgot to mention this when I initially made this post, oops. The mirror in the old mansion had the kanji for The Dragon in the Chinese zodiac. The "time of The Dragon" is 7 to 9 AM, thus 8. The best I could muster was "VIII." Hey, gimme a break.
Submerged Past
The game is somewhat subtle about it, but this seems to be the backstory with Saeko and Yukio.
Saeko was married and had two kids, Chiyo and Chie. Her husband died, possibly in a traumatic way: her letter says she's "on her own now," got compensation for "emotional distress," and the kids needed counseling.
Also, Chiyo and Chie's genders are left ambiguous. But since Chiyo felt bad for the mermaids and drew one crying, while Chie loved them and drew a happy one, it might be assumed Chiyo was a girl and Chie was a boy.
Saeko and Yukio were probably acquaintances. Yukio, worried for her, offered to marry Saeko, or maybe made some other offer which eventually led to that.
However, Saeko soon found out what the men of the family did with the mermaids. Disgusted and regretting ever marrying into the family, she left. (Her stepmother also had thoughts of running away.)
The Tsuchida men wouldn't risk Saeko knowing their secret, however, and drowned Saeko and her children. (A little uncertain, but it seems likely, what with Saeko saying "I don't know" to "Will we be killed?")
Yukio almost certainly learned of the foul play, which probably helped him to realize just how awful his family was. Though since he still had a thing for the mermaids and wouldn't let them go, it didn't exactly help anything.
Mermurderers
It's not too hard to work out, but the mermaids "bloated" Yuka as a means to get the others to find and help them. This is why (dream) Yuka says she's a "hostage" and is returned to normal in the true ending.
Uri confirmed this on Twitter, as well as that the "HELP ME..." and "I'M SORRY..." calls are Yuka speaking for the mermaids: either again asking for help (both for the mermaids and Yuka), or apologizing for their gamble killing Yuka.
Uri also confirmed that the hairpin in the swamp and the clothes in that one room were left behind by the mermaids; the latter probably while they were living among the family, the former falling out while being taken to the other side of the swamp.
The mermaids probably gave Rin weird dreams and such to lead her on the right track, and maybe make her sympathize. Theoretically, Rin and Yuka could have traded places. The mermaids were understandably unwilling to count on the men (smart move).
The insane Yuuta and Seitaro were at least in some sense possessed by the spirits of the Tsuchida men - the ghost behind Seitaro in the graveyard seems to imply it's fairly literal.
They seem quite a bit more misogynistic in crazy mode, too. Again, they love drowning women, so this should not be surprising.