Sethian

Here is an excerpt from Sethian's Steam description:

Sethian is a sci-fi puzzle game in which you master a fictitious language. An archaeologist in the distant future, you venture to the far-flung planet of Sethian, investigating the mysterious disappearance of the people there centuries ago. An abandoned computer may have the answers you seek, but it only operates in the lost native language. Master their language, unravel their mysteries, and challenge their philosophy.

Unlike in other games, Sethianese is a more earnest attempt at creating a fictitious language, using its own unique grammar and vocabulary rather than simply substituting words and letters one-for-one with English. The game's language borrows elements from Chinese, American Sign Language, and other real world languages, in addition to some features which have no real world counterpart.

Assisting you in your investigation are your handwritten journal and dictionary, easing you into the game's bizarre language, providing you with a reliable reference, and introducing you to the literature and lore of the world.