Okay, so when you finish single-player, a monolith appears near the single-player entrance that you can activate to play the credits. I thought that this was going to be a Rosetta stone for the game, since it's got Inkling and you can compare it to the names of the development crew. Turns out that it's either:
1. A red herring and is not meant to actually mean anything, or
2. Evidence that the Inkling language is just gibberish.
I'm not certain which of those two it is at this point, but unfortunately I'm currently leaning toward the idea that it is gibberish. But here's what I found to show that, in the best case scenario, this particular object is not going to help decode the Inkling language. At worst, it's proof that Inkling can't be decoded.
Basically, the monolith is composed of 4 lines of Inkling repeated over and over and over. Lines could be flipped 180 degrees, but otherwise what first looked like it was the credits in Inkling is just the same 4 lines repeated in a bunch of different combinations so that, from a distance, it resembles the credits but is actually just meaningless. The images below are the 4 lines that are repeated, and an image of the monolith. I used several images of the monolith to get a composite view that showed each portion in focus enough to check if there were any regions that were not repeats of one of the four sequences (or upside-down versions of them). It's all the same.
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Also, the reason why I'm leaning toward the idea that Inkling is just gibberish has to do with some of the lines that I see repeated in this monolith are ALSO seen in other places throughout Splatoon. For instance: compare my notes in the previous post to these images taken from warning signs dotting Camp Triggerfish. There are several phrases of three or more characters that are repeated across these three images AND in the monolith.
These images are all warning about hazards or dangers, while the monolith is supposed to be the list of credits. There's very little reason that I can see for why they should contain the same words as one another, unless the words didn't have meaning. Specifically, compare the second bullet point on that warning sign that has lots of text (image below) and the first line of text I wrote on the graph paper (image in the previous post). It's almost identical, and I can't think of a good reason why that would be other than that it's meaningless.
So, with this in mind, I'm currently leaning towards the postulate that Nintendo did not invent some kind of transliterative code that can be decoded to reveal meaning in its signs. Instead, I think Nintendo developed a series of characters that represent the Inkling language, but did not set up any rules about when or how to use those characters, so that instead you end up with some uses of the language where characters are chosen so that it is quasi-readable, and other times it's just a garbled mess of jumbled characters.
That's where I'm leaning toward at the moment. I'm not completely convinced at this point, though. I need to dig a little deeper.