Is it possible to detect the player's Minecraft language in Singleplayer Minecraft (Vanilla). If so, could you give me the command.
That is not possible. What you can do is create language support in resource packs as you can see in maps like The Heist.
In the resource pack go to assets/minecraft/lang folder. Create files like EN_US.lang with content like:
language.name=English language.region=US language.code=en_US container.dropper=Dopfer container.furnace=New funace container.enchant=Book stealer container.crafting= deathScreen.respawn=Restart! deathScreen.title=Game Over
Hope it helped :)