Great question, and I'm glad we can sort this out now early in the proposal.
If we consider the field of medicine, there are many specialties, such as radiology, cardiology, neurology, gastroenterology and so forth. I don't think questions that only a cardiologist can answer however should be off-topic on the medicine proposal. The medicine proposal will hopefully comprise of experts in each of those respective specialties. Similarly, in this proposal, hopefully all experts will be able to provide answers for questions on theories of law, and each expert will have a national specialty, but this doesn't mean they won't be able to assist in questions from foreign nations either, depending on how similar the countries are.
The 193 countries that exist should not deter people from this site. Of these only 57 are English speaking countries. Now even though theoretically we would like to believe we have an even distribution of users from each of those 57 countries, we frankly don't, as most users are from a collection of just a few countries. Now this site will include experts in the theory of law, and also experts in the laws of their individual countries. But often an expert in Australian law may often know about UK laws or be able to use their expertise to find out quickly. So this proposal is no different to other proposals that have specialized fields.