Hey, I tried it (by the way, is it just me or Itch.io download speed is very slow? It took me a good 15 minutes to download the demo).
The visuals are very nice, so props on that one. I loved how you could see the stars moving, and how the background moon (or planet?) shadow was moving as well, that was a nice touch.
Language selection worked fine for me, but you showed both languages in french ("français", "anglais"). Wouldn't be better to show the names on its native language? ("français", "english", "español", etc.)
Now, regarding gameplay, I must admit that I didn't understand it. You can modify the terrain by using S or F, but I didn't find a pattern, I was just pressing the keys randomly until I got a terrain on which could pass through. Maybe I'm missing something?