The Montreal convention allows enforcement through the courts. It is also incorporated in the airlines' tariffs, so it is currently the option that offers the most tools for enforcement.
It does not carry penalties. It does not have specific enforcement mechanisms. What we do see in claims relating to the Montreal convention is that the passenger makes a claim for a delay in their flight and gets back an email from Air Canada, for example, thanking them for their email and saying, “Here's 25% off your next flight.” They're completely ignoring the substance of the complaint.
Overall, the picture we see with both the Montreal convention and the tariff system is that the rules are written down there, but airlines are training their lower-level staff to ignore complaints of this nature and provide template answers that essentially are evasive.