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

The trap: Implementing runtime language switching when system language suffices. It adds complexity and can break third-party SDKs that read system locale.

String Key Architecture

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"

Pluralization Complexity

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

RTL Support Level

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

NEVER Do

String Management

NEVER concatenate localized strings:

// ❌ 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)

NEVER embed numbers in translation keys:

// ❌ 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
)

NEVER assume string length:

// ❌ 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)

NEVER use left/right in layouts:

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

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

Runtime Language

NEVER change AppleLanguages without restart:

// ❌ 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

NEVER forget to set locale for formatters:

// ❌ 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

Pluralization

NEVER use simple if/else for plurals:

// ❌ 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

NEVER hardcode numeral systems:

// ❌ 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"

RTL Layouts

NEVER use fixed directional icons:

// ❌ 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)

NEVER force layout direction globally when it should be per-component:

// ❌ 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)
}

Essential Patterns

Type-Safe Localization with SwiftGen

// 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

Stringsdict for Plurals

<!-- 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"

RTL-Aware Layout Helpers

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
        }
    }
}

Locale-Aware Formatting

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)"
    }
}

Quick Reference

Plural Forms by Language

Language	Forms	Example (1, 2, 5)
English	one, other	1 item, 2 items, 5 items
French	one, other	1 élément, 2 éléments
Russian	one, few, many, other	1 файл, 2 файла, 5 файлов
Arabic	zero, one, two, few, many, other	6 forms!
Japanese	other only	No grammatical plural

RTL Languages

Language	Script Direction	Numerals
Arabic	RTL	Arabic-Indic (٠١٢) or Western
Hebrew	RTL	Western
Persian	RTL	Extended Arabic (۰۱۲)
Urdu	RTL	Extended Arabic

String Expansion Guidelines

Source (English)	Expansion
1-10 chars	+200-300%
11-20 chars	+80-100%
21-50 chars	+60-80%
51-70 chars	+50-60%
70+ chars	+30%

Red Flags

Smell	Problem	Fix
String concatenation	Word order varies	Format strings
if count == 1 else	Wrong plural rules	stringsdict
.padding(.left)	Breaks RTL	.padding(.leading)
DateFormatter without locale	Wrong language	Set locale explicitly
Runtime language without restart	Inconsistent UI	Require restart
Fixed frame widths for text	Text truncation	Flexible layouts
Hardcoded "1, 2, 3"	Wrong numeral system	NumberFormatter with locale

localization-ios

Localization iOS — Expert Decisions

Decision Trees

Runtime Language Switching