Why not both? You can write config-looking Kotlin code so that user won't even realize it's Kotlin. But type-safe and with auto-completion.

Something like

    server {
      port = 8080
      addr = "127.0.0.1"
      development = true
    }
    
    db {
      url = $DB_URL
      user = "app"
      password = include(".secrets/dbpass.txt")
    }