Here you can find the mapfixes up to map 83, it's not necessary for most and I'm not sure if later ones in the full version still need fixing though.
https://f95zone.to/threads/war-demon-kirstin-demo-captain-sikorsky.99935/post-7202624
Download the files and extract them files in the appropriate locations.
Alternatively here's a 1.41 version of the game already fogfixed with the patch above. I played until the elven village with absolutely no issues. No idea if you have to extract it in japanese, I just packed and unpacked in 7zip, maybe the packing broke something, maybe not.
You must be registered to see the links
Don't get scammed by people saying MTLing is a lot of work when you can literally get the fixes here for free and MTLing is a matter of 3-5 minutes.
At this point you might want to consider just waiting for the proper translation by
Daedroth that is coming out soon though.
He already uploaded a partial translation patch on mega.nz here:
https://f95zone.to/threads/war-demon-kirstin-demo-captain-sikorsky.99935/post-7682632
There are a lot of options in this thread, just pick one you like. Frankly, I'm just waiting for the manual translation by Daedroth to be finished, since it won't take long and I'd rather not have to guess what characters are saying as you often have to with MTLs.
Out of the translations posted here Daedroth's partial currently sounds like the best, but I haven't tried it myself yet.
If you want to manually fix the maps just throw the map.json files into an online json editor, cntrl+f "fog" or "0フォグ3" whatever works and rename it to something the game recognizes in the parallaxes folder, whether you first follow this guide:
https://f95zone.to/threads/war-demon-kirstin-demo-captain-sikorsky.99935/post-6959542
to rename the fog files and then replace the "fog" mentions in the jsons with whatever you chose or whether you copy-paste the original japanese text for the fog mentions in the jsons is up to you. I've seen " Fog 3" to "fog3" fixing it before in the thread, just gotta try things out, it's been too long since I did it myself to remember.
EDIT: Sent the post a bit too early so the patched 1.41 version will be added in a few minutes when the upload is done.
EDIT2: Added the missing link.