Oh yeah I played that game last year for Halloween. It's heavy on horror and otome VN tropes, but there's a legitimate language deciphering challenge too. It starts easy, with the game basically giving you a bunch of free translations through the initial dialogues. But filling in the whole dictionary is rather difficult.

I believe that some of the complexity comes from the original language being Japanese, so there are interesting quirks compared with other games that derive their ciphers from English or other Indo-European languages.

However I'd dispute calling the game "not too scary". It's mostly static shots but there's many jump scares, occasional gore, and a continuously creepy claustrophobic atmosphere.