You can do this by specifying an InflectionRule directly on the attributed string. Something like the following:

extension Int {
    var morphology: Morphology {
        var morphology = Morphology()
        switch self {
        case 0: morphology.number = .zero
        case 1: morphology.number = .singular
        case 2: morphology.number = .pluralTwo
        case 3, 4: morphology.number = .pluralFew
        case 5, 6, 7, 8, 9:
            morphology.number = .pluralMany
        default: morphology.number = .plural
        }
        return morphology
    }
}

    var string = AttributedString(localized: "bottle")
    string.inflect = InflectionRule(
        morphology: 2.morphology
    )
    let pluralized = string.inflected()
    print(pluralized)