This decrypts and decompresses the sl2 files:

{{{#!python

import os, sys, zlib
from rijndael import Rijndael
from struct import unpack

for fname in sys.argv[1:]:
    key = [0x04, 0x38, 0x04, 0x31, 0x2D, 0x32, 0x0C, 0x30, 0x43, 0x2E, 0x08, 0x04, 0x16, 0x30, 0x22, 0x0C]
    iv  = [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]
    with open(fname, "rb") as f:
        with open(fname.replace(".sl2", "")+".txt", "wb") as f2:
            print(f"handling {fname}")
            rjn = Rijndael(key, block_size = 16)
            fdata = f.read()
            
            plaintext_len = unpack(">I", fdata[0x4C:0x50])[0]
            ciphertext_len = (plaintext_len//16) * 16
            if ciphertext_len < plaintext_len:
                ciphertext_len += 16
            
            data = fdata[0x50:ciphertext_len+0x50]
            
            decrypted = []
            while len(data) >= 16:
                ciphertext = data[:16]
                plaintext = rjn.decrypt(ciphertext)
                for i in range(len(plaintext)):
                    plaintext[i] ^= iv[i]
                decrypted += plaintext
                iv = ciphertext
                data = data[16:]
            decrypted = decrypted[:plaintext_len]
            
            if True:    
                decompressed = zlib.decompress(bytes(decrypted))
            
                string_len = unpack(">I", bytes(decompressed[0x2C:0x30]))[0]
                text = decompressed[0x30:0x30+string_len-1]
            
                f2.write(text)
            else:
                f2.write(bytes(decrypted))
}}}

This parses them:

{{{#!python

import os, sys, json, re

for fname in sys.argv[1:]:
    with open(fname, "r", encoding="cp932") as f:
        lines = []
        
        parsing_if = -1
        data = json.load(f)
        i = 0
        while len(data[i:]) > 0:
            entry = data[i]
            if parsing_if == 1:
                if entry != "true":
                    print(entry)
                    print("^-- expected \"true\" block of if statement")
                    exit()
                else:
                    parsing_if = 2
                    i += 1
                    continue
            if parsing_if == 2:
                if entry == "false":
                    parsing_if = 3
                    i += 1
                    continue
            if parsing_if == 3:
                if entry == "endif":
                    parsing_if = -1
                    i += 1
                    continue
            if isinstance(entry, str):
                if entry == "_if":
                    if parsing_if != -1:
                        print("error: nested if statements not supported")
                    parsing_if = 1
                    i += 2
                    continue
                
                if entry[0] in "_":
                    # wild guess lol
                    if len(data[i:]) > 1 and not isinstance(data[i+1], str):
                        i += 2
                    else:
                        i += 1
                    continue
                if entry[0] in "#":
                    i += 1
                    continue
                if entry in ["ending"]:
                    i += 1
                    continue
                if entry in ["false", "true", "endif"]:
                    print("in: "+fname)
                    print(parsing_if)
                    print("")
                    print(data[i-6])
                    print(data[i-5])
                    print(data[i-4])
                    print(data[i-3])
                    print(data[i-2])
                    print(data[i-1])
                    print(data[i] + " <---!!")
                    print(data[i+1])
                    print(data[i+2])
                    print(data[i+3])
                    print(data[i+4])
                    print(data[i+5])
                    print(data[i+6])
                    exit()
                if entry[0] in "qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM_#":
                    print(data[i-6])
                    print(data[i-5])
                    print(data[i-4])
                    print(data[i-3])
                    print(data[i-2])
                    print(data[i-1])
                    print(data[i] + " <---()")
                    print(data[i+1])
                    print(data[i+2])
                    print(data[i+3])
                    print(data[i+4])
                    print(data[i+5])
                    print(data[i+6])
                    exit()
                lines += [entry]
                i += 1
            else:
                print(entry)
                print("^-- unexpected list")
                exit()
        
        lastline = ""
        for line in lines:
            if line[0] != '/':
                if lastline.endswith("」"):
                    lastline = re.sub(r"[^「]*(「.*」)", r"\1", lastline)
                print(lastline)
                lastline = line
            else:
                lastline += line[1:]
            #if line.endswith("」"):
            #    line = 
}}}

rijndael.py is here: https://gist.github.com/wareya/e00c80c9d346312354d45516978e3376