Making a new protocol with the correct required method and extension, and then extending LocalizedStringKeys/String.StringInterpolation with this new protocol seems to work for me:
protocol Formattable {
mutating func appendInterpolation(_: String)
}
extension Formattable {
mutating func appendInterpolation(_ value: Double, format: String) {
appendInterpolation(String(format: format, value))
}
}
extension LocalizedStringKey.StringInterpolation: Formattable {}
extension String.StringInterpolation: Formattable {}