International treaties do not automatically trump national law. A treaty can be broken with consequences.

However no international law should have moral authority without due representation. It is a broken system when democratic governments kowtow to non democratic international bodies. Yes it is possible to have benign dictatorships. Courts of Law are a bit different as we normally do not elect judges, we elect the law makers instead.

As for civil vs common. I suspect it can be a bit more messy in some areas where assumed knowledge is left. It would also in my eyes require more verbiage to describe the same Act so as to cover both civil and common law interpretations.

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As for the US it is not able for an international treaty to trum its p national laws as that is part of their Consitution.