I think it would be great to have a way to work through these challenges using a language that isn’t “officially” supported, like Swift. As a simple implementation, the challenge would provide a set of tests with expected results for each stage, and it would be up to the user to ensure that the tests are passing before moving on to the next stage.
This would provide incentive for an interested user to sign up (or renew), but is reluctant because their language of choice isn’t supported.