Have you ever heard of the Latin phrase: Pacta Sunt Servanda (Agreement must be kept) ? It is a basic principle that "Every treaty in force is binding upon the parties to it and must be performed by them in good faith."(Article 26 of the Vienna Convention on the Law of Treaties 1969) A treaty means "an international agreement concluded between States in written form and governed by international law..."(Article 2)
Is a (joint) declaration a treaty then? While declarations generally express certain non-binding aspirations without ratification provisions, a formal agreement with respect to a matter of major importance may be binding, despite its unusual nomenclature. Eg, para 8 of the Sino-British Joint Declaration on the Question of Hong Kong 1984 provides for ratification and para 7 declares that both United Kingdom and Chinese Governments "agree to implement the preceding declarations and the Annexes to this Joint Declaration."
But who or what institution has the authority to make such a good or correct treaty interpretation? The international judicial system is consensual in nature and in the absence of any authentic third-party dispute settlement mechanism, it is up to the parties themselves to do their respective "auto-interpretations" in case of conflicts. Treaty interpretation might then become an art ie in the eyes of the beholders.
I believe that States run by humans are generally interest-based and rational, and should be concerned about direct or indirect sanctions and their reputational risk should they act arbitrarily. Practising the art of treaty interpretation would still be principled, basing on international legal standards and norms. In the imperfect legal order, pacta sunt servanda provides the jurisprudential basis for international compliance and good governance!