Generally treaties are negotiated by the executive of a country, not the legislative force. Even in the cases where there is still a ratification step, the initiative is taken away from the legislative, and the law is written by indirectly (at best) elected commissars (in the case of the EU), rather than directly elected representatives.

In the case of the EU, there isn't even a ratification step once the EU commission (which isn't elected) agrees to a treaty.

And of course ... https://en.wikipedia.org/wiki/Supremacy_(European_Union_law)

That means that with EU commission agreement you get the supreme court (that supersedes all local courts) arguing that anything the commission agrees to can countermand local constitutions.

So local legislature agreement required, in many cases, is incorrect.

Eh, no it isn't (necessarily). That depends on whether a certain legal system is monistic or dualistic; or rather, adheres to monistic or dualistic principles (it's not a black or white thing). Without knowing where the GP is from, you can't reasonably make any assumptions, because both are common.