I'm so glad you like it so much, oh my gosh! :D
Yes, I did make up the language myself! And I'd love to see your decipherings of it! Maybe just put a potential spoiler warning at the beginning of the comment? :0 But I would love to see!
[SPOILER WARNING]
Alrighty then!
So I think there are four images in total that contain these messages. It's certainly possible I missed some though! Anyway, I believe the mystery grave says something like: "Rest in pain, forever missing" (I was expecting there to be a name on the grave, it's kind of interesting that Mary calls this person a "they" and Reginald calls them a "him", though, is that intentional?).
After the argument with Twyla, when Mary hides behind a trashcan in an alley. I think the graffiti says something like: "Silly girl".
When Mary runs away from Reginald in his apartment complex, I believe the tags on the doors say "Run".
And lastly, I dont really know what the word in the creepy manga is supposed to be. My closest guess would probably be "Wised", but I'm not so sure. Maybe it's a name?
Wow, you translated them pretty well I'd say! I'm impressed! :D Except for the manga one--just a bit off! But you're close, I think!
And as for the grave...hehe. Maybe it's a bit intentional! ;-) I do intend for some of these characters to reappear in future projects, though! So maybe the puzzle will begin to reveal itself more and more as that happens.
I feel so flattered you enjoyed the game enough to want to decipher the codes! Thank you so much for playing, I hope you'll stick around for future projects too!<3