This is covered in the film's IMDb FAQ:
When Karol watches Dominique with the binoculars, she uses the sign language saying, ("quand je me libère de prison, toi et me, on part ensemble, oui? Ou on reste ici, ensemble, on se marie.") "When I get out of prison, you and me, we'll leave together, OK? Or we'll stay here together and get married again," (as Domique shows the act of putting a ring on the left hand on the ring finger), explained by Julie Delpy in [the] bonus feature of the DVD.
Here's Delpy explaining exactly what she said in that scene and why: