Do users need in-app language control?
├─ NO (respect system language)
│  └─ Standard localization
│     NSLocalizedString, let iOS handle it
│     Simplest and recommended
│
├─ YES (business requirement)
│  └─ Does app restart work?
│     ├─ YES → UserDefaults + AppleLanguages
│     │  Simpler, more reliable
│     └─ NO → Full runtime switching
│        Complex: Bundle swizzling or custom lookup
│
└─ Single language override only (e.g., always English)
   └─ Don't localize
      Bundle.main with no .lproj

How to structure your keys?
├─ Small app (< 100 strings)
│  └─ Flat keys with prefixes
│     "login_title", "login_email_placeholder"
│
├─ Medium app
│  └─ Hierarchical dot notation
│     "auth.login.title", "auth.login.email"
│
├─ Large app with teams
│  └─ Feature-based files
│     Auth.strings, Profile.strings, etc.
│     Each team owns their strings file
│
└─ Design system / component library
   └─ Component-scoped keys
      "button.primary.title", "input.error.required"

Which languages do you support?
├─ Western languages only (en, es, fr, de)
│  └─ Simple plural rules
│     one, other (maybe zero)
│
├─ Slavic languages (ru, pl, uk)
│  └─ Complex plural rules
│     one, few, many, other
│     e.g., Russian: 1 файл, 2 файла, 5 файлов
│
├─ Arabic
│  └─ Six plural forms!
│     zero, one, two, few, many, other
│     MUST use stringsdict
│
└─ East Asian (zh, ja, ko)
   └─ No grammatical plural
      But may need counters/classifiers

Do you support RTL languages?
├─ NO RTL languages planned
│  └─ Still use leading/trailing
│     Future-proof your layout
│
├─ Arabic only
│  └─ Standard RTL support
│     layoutDirection + leading/trailing
│     Test thoroughly
│
├─ Arabic + Hebrew + Persian
│  └─ Each has unique considerations
│     Hebrew: different number handling
│     Persian: different numerals (۱۲۳)
│
└─ Mixed LTR/RTL content
   └─ Explicit direction per component
      Force LTR for code, URLs, numbers

// ❌ Breaks in languages with different word order
let message = NSLocalizedString("hello", comment: "") + " " + userName

// German: "Hallo" + " " + "Hans" = "Hallo Hans" ✓
// Japanese: "こんにちは" + " " + "田中" = "こんにちは 田中" ✗
// Should be "田中さん、こんにちは"

// ✅ Use format strings
let format = NSLocalizedString("greeting.format", comment: "")
let message = String(format: format, userName)
// greeting.format = "Hello, %@!" (en)
// greeting.format = "%@さん、こんにちは!" (ja)

// ❌ Doesn't handle plural rules
"items.1" = "1 item"
"items.2" = "2 items"
"items.3" = "3 items"
// What about 0? 100? Arabic's 6 forms?

// ✅ Use stringsdict for plurals
String.localizedStringWithFormat(
    NSLocalizedString("items.count", comment: ""),
    count
)

// ❌ German is ~30% longer than English
.frame(width: 100)  // "Settings" fits, "Einstellungen" doesn't

// ✅ Use flexible layouts
.frame(minWidth: 80)
// Or
.fixedSize(horizontal: true, vertical: false)

// ❌ Breaks in RTL
.padding(.left, 16)
.frame(alignment: .left)

// ✅ Use leading/trailing
.padding(.leading, 16)
.frame(alignment: .leading)

// ❌ Partial UI update — inconsistent state
UserDefaults.standard.set(["ar"], forKey: "AppleLanguages")
// Some views updated, others not. Third-party SDKs broken.

// ✅ Require restart or use custom bundle
UserDefaults.standard.set(["ar"], forKey: "AppleLanguages")
showRestartRequiredAlert()  // User restarts app

// ❌ Uses device locale, not app's selected language
let formatter = DateFormatter()
formatter.dateStyle = .medium
let date = formatter.string(from: Date())  // Wrong language!

// ✅ Set locale explicitly
let formatter = DateFormatter()
formatter.locale = Locale(identifier: selectedLanguage.rawValue)
formatter.dateStyle = .medium

// ❌ Fails for Russian, Arabic, etc.
func itemsText(_ count: Int) -> String {
    if count == 1 {
        return "1 item"
    } else {
        return "\(count) items"
    }
}

// Russian: 1 товар, 2 товара, 5 товаров, 21 товар, 22 товара...
// This requires CLDR plural rules

// ✅ Use stringsdict — iOS handles rules automatically

// ❌ Arabic users may expect Arabic-Indic numerals
Text("\(count) items")  // Shows "5 items" even in Arabic

// ✅ Use NumberFormatter with locale
let formatter = NumberFormatter()
formatter.locale = Locale(identifier: "ar")
formatter.string(from: count as NSNumber)  // "٥"

// ❌ Arrow points wrong way in RTL
Image(systemName: "arrow.right")

// ✅ Use semantic icons or flip
Image(systemName: "arrow.forward")  // Semantic
// Or
Image(systemName: "arrow.right")
    .flipsForRightToLeftLayoutDirection(true)

// ❌ Phone numbers, code, etc. should stay LTR
.environment(\.layoutDirection, .rightToLeft)

// ✅ Apply selectively
VStack {
    Text(localizedContent)  // Follows RTL

    Text(phoneNumber)
        .environment(\.layoutDirection, .leftToRight)  // Always LTR

    Text(codeSnippet)
        .environment(\.layoutDirection, .leftToRight)
}

// swiftgen.yml
// strings:
//   inputs: Resources/en.lproj/Localizable.strings
//   outputs:
//     - templateName: structured-swift5
//       output: Generated/Strings.swift

// Usage — compile-time safe
Text(L10n.Auth.Login.title)
Text(L10n.User.greeting(userName))
Text(L10n.Items.count(itemCount))

// Benefits:
// - Compiler catches missing keys
// - Auto-complete for strings
// - Refactoring safe

<!-- en.lproj/Localizable.stringsdict -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" ...>
<plist version="1.0">
<dict>
    <key>items.count</key>
    <dict>
        <key>NSStringLocalizedFormatKey</key>
        <string>%#@items@</string>
        <key>items</key>
        <dict>
            <key>NSStringFormatSpecTypeKey</key>
            <string>NSStringPluralRuleType</string>
            <key>NSStringFormatValueTypeKey</key>
            <string>d</string>
            <key>zero</key>
            <string>No items</string>
            <key>one</key>
            <string>%d item</string>
            <key>other</key>
            <string>%d items</string>
        </dict>
    </dict>
</dict>
</plist>

// Usage
let text = String.localizedStringWithFormat(
    NSLocalizedString("items.count", comment: ""),
    count
)
// 0 → "No items"
// 1 → "1 item"
// 5 → "5 items"

extension View {
    /// Applies leading alignment that respects RTL
    func alignLeading() -> some View {
        self.frame(maxWidth: .infinity, alignment: .leading)
    }

    /// Force LTR for content that shouldn't flip (code, URLs, phone numbers)
    func forceLTR() -> some View {
        self.environment(\.layoutDirection, .leftToRight)
    }
}

// ContentView
struct MessageCell: View {
    let message: Message

    var body: some View {
        VStack(alignment: .leading) {
            Text(message.content)
                .alignLeading()  // Respects RTL

            Text(message.codeSnippet)
                .font(.monospaced(.body)())
                .forceLTR()  // Code always LTR

            Text(message.url)
                .forceLTR()  // URLs always LTR
        }
    }
}

struct LocalizedFormatters {
    let locale: Locale

    init(languageCode: String) {
        self.locale = Locale(identifier: languageCode)
    }

    func formatDate(_ date: Date, style: DateFormatter.Style = .medium) -> String {
        let formatter = DateFormatter()
        formatter.locale = locale
        formatter.dateStyle = style
        return formatter.string(from: date)
    }

    func formatNumber(_ number: Double) -> String {
        let formatter = NumberFormatter()
        formatter.locale = locale
        formatter.numberStyle = .decimal
        return formatter.string(from: NSNumber(value: number)) ?? "\(number)"
    }

    func formatCurrency(_ amount: Double, code: String) -> String {
        let formatter = NumberFormatter()
        formatter.locale = locale
        formatter.numberStyle = .currency
        formatter.currencyCode = code
        return formatter.string(from: NSNumber(value: amount)) ?? "\(amount)"
    }
}