You measure it in the same way, using the idea of shortest decription w.r.t. a fixed universal language. You than show that this is absolute up to a constant. A

You measure it in the same way, using the idea of shortest decription w.r.t. a fixed universal language. You than show that this is absolute up to a constant. Any results produced that way such as the incompressibility of most strings is independent of such constants, hence the the choice of universal language doesn't matter.