First things first, this language is a very straightforward kana-only Japanese in a different script. I started with the text under each main menu option, so let's look at these:

"Press any button" はじめる Hajimeru Start

The : mark next to the second character functions just like dakuten—the base character is し (shi), which becomes voiced to make じ (ji).

"New Game" はじめから Hajime kara From start

"Continue" つづきから Tsuzuki kara From next

"Load" きろくから Kiroku kara From

Pretty straightforward—I got stuck here for way too long because I thought these were -masu verbs and then nothing else made sense, oops.

"Gallery" せってい Settei Settings

"Settings" がろう Garou Gallery

"End Game" しゅうりょう Shuuryou End

Another sticking point here—the text on settings and gallery are actually switched. One danger of putting a fake language in your game is that you risk not being able to read it and messing it up in places (hi Metaphor), and though it's not really critical, this is not the only goof so far. (EDIT: This has been patched!)

Note that the smaller characters here are are reflected in the script as well, like the っ in せってい.

Cutscene dialogue

The subtitles for our commander's transformation cutscene, and a rough translation:

いだいなほしのかみよ… Idai na hoshi no kami yo... God of a great star(/planet)...

われにちからをあたえよ…！ Ware ni chikara wo atae yo...! Grant me power...!

むげんたい…！！ Mugentai...!! Eternal body...!!

ぼくは…「ちつじょのけしん」むゔゔむ… Boku wa... [Chitsujo no Keshin] Muvuvumu... I am... the [Avatar of Order] Muvuvumu... (MVVM?)

This was the section I struggled the most with, so there could be inaccuracies, but I'm reasonably confident in most of it.

Post-Battle Dialogue

Here's the very straightforward part:

こ、ころさないで… Ko-korosanaide... D-Don't kill me...

ゆるしてください Yurushite kudasai... Please forgive me...

おねがい…たすけて… Onegai... tasukete... Please... spare me... (or help me, but spare felt most fitting in this context)

And that's about everything we have so far! Here are my very janky kana charts, one for the script and one for the spoken language:

The most obvious observation is that the general shape seems to indicate the vowel sound, and the extra swoopy bits indicate the consonant, though some vowels are more consistent than others.

There's a general pattern among each consonants as well—N-shapes have loops or curls, S-shapes have angular lines that hook over the vowel shape, pure vowels have tiny loops, etc, but there aren't clear-cut enough patterns to extrapolate the rest of the language yet. Luckily I think there's enough so far that any remaining characters won't be too hard to place.

Let's talk about the spoken component as well! Notably, the commander is dubbed differently in English and Japanese, but both voice actors seem to be reading the same set of syllables, which gave me more sample data to work with, because they inflect their vowels a little differently. Like the written language, each kana has a 1:1 replacement syllable, but the nature of these syllables changes the cadence of the sentences when spoken.

The above chart is transcribed to the best of my ability, but it's possible I got a few sounds wrong, and I can't quite make out what the tsu in chitsujo is through the voice filter. I'm considering replaying the demo with the music and sound effects off to pick these voice lines out, and if I do, I'll edit this post and drop a reply noting it! There are also some unsubbed battle voice lines I'm curious about.

What's really interesting to me is how these new syllables interact with dakuten. Plenty of unvoiced consonants are replaced with voiced ones, or the other way around—but there are a few, like ta (now tu) and te (now se). When the Japanese script uses dakuten, the new syllables get voiced too, becoming du and ze. This tripped me up a bit, and it does cause some confusing duplicates (ku is already pronounced du), but it's fascinating that it bleeds through into the new pronunciation.

One other thing I noticed when analyzing these voice lines is that the English dub of the transformation cutscene straight up uses the wrong audio clip for the final line. The Japanese audio matches the syllable count, and those syllables match up with the ones we've heard in the other voice lines, but the English line seems to have too many syllables, and doesn't match at all. JP starts with "diduwa..." and EN starts with something like "mozoku en." I'm wondering if it's possible to reverse engineer this to figure out the original line, but that's another thing that would need scrutiny from a voice-only recording.

Misc. Observations

These little guys have the character for "o" on their faces! Also pronounced "ga" in the invaders' language.

I'm really intrigued by this graffiti on the walls when Takumi chases Karua into the abandoned school district, because this game is full of Japanese graphic text in the environments, so this can't be a case of generic unreadable background text—but it doesn't quite fit the invader script, either. A few shapes sort of match the characters if you rotate or invert them different ways, but I can't parse anything readable out of this. There are repeated shapes, like the topmost "word" in the first two images that appears to be the same, but written with slightly different strokes, so I have to assume there's meaning to it. Not sure what that is, but I wanted to include it in this post just in case.