Using the AppStoreConnect_Swift_SDK package to update localizations on App Store Connect.

UpdateLocalizations.swift

import AppStoreConnect_Swift_SDK

func updateMetadata(forBundleID bundleID: String) async throws {
    let configuration = try! APIConfiguration(issuerID: "...", privateKeyID: "...", privateKey: "...")
    let provider = APIProvider(configuration: configuration)
    
    // Get app
    let appRequest = APIEndpoint.v1
        .apps
        .get(parameters: .init(filterBundleID: [bundleID], include: [.appInfos, .appStoreVersions]))
    let app: App = try! await provider.request(appRequest).data.first!
    
    // Get latest version
    let versionId = app.relationships!.appStoreVersions!.data!.first!.id
    let versionRequest = APIEndpoint.v1
        .appStoreVersions
        .id(versionId)
        .get(parameters: .init(include: [.appStoreVersionLocalizations], limitAppStoreVersionLocalizations: 50))
    let version: AppStoreVersion = try! await provider.request(versionRequest).data
    
    // Get localizations
    let localizations = version.relationships!.appStoreVersionLocalizations!.data!
    for localizationId in localizations.map({ $0.id }) {
        // Get localization
        let localizationRequest = APIEndpoint.v1
            .appStoreVersionLocalizations
            .id(localizationId)
            .get()
        let localization: AppStoreVersionLocalization = try! await provider.request(localizationRequest).data
                            
        // Read strings to upload
        let locale = localization.attributes!.locale!
        let releaseNotes = "Release Notes for What's New for \(locale)" // Get string for locale
        let promotionalText = "Promotional for \(locale)" // Get string for locale
        
        // Update localization
        let updateLocalizationRequest = APIEndpoint.v1
            .appStoreVersionLocalizations
            .id(localizationId)
            .patch(.init(data: .init(type: .appStoreVersionLocalizations, id: localizationId, attributes: .init(promotionalText: promotionalText, whatsNew: releaseNotes))))
        let _: AppStoreVersionLocalization = try! await provider.request(updateLocalizationRequest).data
    }
    
}

As an aside, I wish someone would just write an API that works like this:

import AppStoreConnect

let app = App(bundleId: "...")
for localization in await app.versions.first!.localizations {
    await localization.update(whatsNew: "String for \(localization.locale)")
    await localization.update(promotionalText: "String for \(localization.locale)")
}

Done. Now that's an API!