Format string with variadic arguments

String has two similar initializers:

init(format: String, _ arguments: CVarArg...)
init(format: String, arguments: [CVarArg])

The first one takes a varying number of arguments, the second one an array with all arguments:

print(String(format: "x=%d, y=%d", 1, 2))
print(String(format: "x=%d, y=%d", arguments: [1, 2]))

In your localized method, args: CVarArg... is a variadic parameter and those are made available within the functions body as an array of the appropriated type, in this case [CVarArg]. Therefore it must be passed to String(format: arguments:):

func localized(table: String? = nil, bundle: Bundle = .main, args: CVarArg...) -> String {
  return String(
    format: NSLocalizedString(
      tableName: table,
      bundle: bundle,
      value: self,
      comment: ""
    arguments: args   // <--- HERE

See also "Variadic Parameters" in the "Functions" chapter of the Swift reference.