Foreign-language Rust forums are a good idea.
I don't know the answer to this off hand, but obviously this is something the Rust project should have an opinion on. The core team will probably want to discuss internally.
Right now management of official Rust domains is a mess. Semi-random people own everything. We probably shouldn't do any transfers without doing some thinking about how we manage domains.
I suspect that right now, taking on any sort of responsibility for managing more forums isn't something the project itself wants to do, and using the rust-lang.* domain does imply a certain amount of officiality.