But there's already multiple existing configuration languages that's far more legible and robust than custom languages implemented on top of XML. Take Nickel.

This:

    let
      totalOwed = totalTax - totalPayments,
      totalTax = tentativeTaxNetNonRefundableCredits + totalOtherTaxes,
      totalPayments = totalEstimatedTaxesPaid +
                      totalTaxesPaidOnSocialSecurityIncome +
                      totalRefundableCredits,
    in
    totalPayments

is easy to read, unlike XML. It's written in a small configuration language that's easy to learn. It's pure and declarative. It handles complex configurations well. It provides tools to quickly pinpoint configuration errors. It can be integrated into existing software and workflows. Compared to bespoke languages built on top of XML, it's an improvement in every way conceivable.

There are also varieties of other languages to choose from. Using a bespoke XML-based language will inflict needless suffering upon people.