Yes, Zotero isn't set up for the very complex structures of citation of international legal materials! If you are working in international law, there is a project called JurisM which builds on Zotero for those working in legal fields (esp international and comparative). It used to be a version I used, but it's changed a lot since I stopped using it: https://juris-m.github.io/

If you have just have a few here and there, then I use a combination of statute (for treaties) and reports for other items. You can usually figure out a way to get the fields to display close enough to what you want. For complex styles like OSCOLA, it won't do it perfectly, but you can do two things - you can edit them directly using the Add Classic Citation dialog (might be able to do it from the default one too, but I don't use that) using the 'Show editor' to display how you want, or you can just fix them at the end after de-linking the citations from Zotero if you want.