What are some undecidable problems for recursive languages?
For recursive languages, membership is decidable by definition, so most undecidable questions with regards to them will involve quantifiers. It's decidable to check if there's a given input is accepted, but it's undecidable to check whether a recursive language accepts any string, for example.

That said, questions like this are almost never phrased in terms of recursive languages, because there's representation of languages that captures exactly the recursive languages. Generally we represent languages as Turing Machines, but these characterize the recursively enumerable languages.

When it comes to RE, there's something called Rice's theorem,which says that any non trivial property of RE languages is undecidable. This holds for anything that's a property of the language itself, not just the specific representation.