Sinatra doesn’t know this ditty.

Try this:
# in main.rb
class GrapesRV
  get '/%D0%BD%D0%B0%D1%81%D1%82%D1%80%D0%BE%D0%B9%D0%BA%D0%B0_%D0%BA%D0%BB%D0%B8%D0%B5%D0%BD%D1%82%D0%BE%D0%B2' do
    "Hello World"
  end
end