I am reading a book that explains Gödel's proofs of incompletenss, and I found something that disturbs me. There is a hidden premise that says something like "all statements of number theory can be expressed as Gödel numbers". How exactly do we know that? Can that be proved? The book did give few examples of translations of such kind (for example, how to turn statement "there is no largest prime number" into statement of formal system that resembles PM, and then how to turn that into Gödel number).
So the question is: how do we know that every normal-language number theory statement has its equivalent in formal system such as PM? (it does seem intuitive, but what if there's a hole somewhere?)