iOS Localization & Internationalization

Localize Apple-platform apps with String Catalogs, modern string types, locale-aware formatting, and right-to-left layout.

Contents

• String Catalogs and Generated Symbols
• String Types -- Decision Guide
• String Interpolation in Localized Strings
• Pluralization
• FormatStyle -- Locale-Aware Formatting
• Right-to-Left (RTL) Layout
• Common Mistakes
• Localization Review Checklist
• References

String Catalogs and Generated Symbols

String Catalogs are the recommended Xcode 15+ workflow for new localization work. They keep localizable strings, pluralization rules, and device variations together in an Xcode-managed JSON file with a visual editor. Legacy .strings and .stringsdict files can coexist during migration, but new Swift and SwiftUI code should default to String Catalogs.

How automatic extraction works:

Xcode scans for these patterns on each build:

// SwiftUI -- automatically extracted (LocalizedStringKey)
Text("Welcome back")              // key: "Welcome back"
Label("Settings", systemImage: "gear")
Button("Save") { }
Toggle("Dark Mode", isOn: $dark)

// Programmatic -- automatically extracted
String(localized: "No items found")
LocalizedStringResource("Order placed")

// Plain String: not extracted or localized
let msg = "Hello"

Xcode adds discovered keys to the String Catalog automatically. Mark translations as Needs Review, Translated, or Stale in the editor.

For detailed String Catalog workflows, migration, and testing strategies, see references/string-catalogs.md.

Generated symbols are an Xcode 26 typed-access layer on top of String Catalogs; they do not change the catalog's Xcode 15 availability.

Enable: Build Settings > Localization > Generate String Catalog Symbols → Yes (on by default in new Xcode 26 projects). Requires catalog format version 1.1.

Workflow: Add a key manually via the (+) button in the String Catalog editor — manual keys have the Generate Swift Symbol checkbox enabled by default. Auto-extracted keys can also opt in via Refactor > Convert Strings to Symbols. Use stable manual keys for generated-symbol strings. Avoid source-copy-derived keys for API-facing strings because wording edits can rename generated identifiers and churn call sites.

// Generated from key "room_available" in Localizable.xcstrings
Text(.roomAvailable)

// Parameterized key "landmarks_count" with %1$(count)lld
Text(.landmarksCount(count: 42))

// Non-default table "Booking.xcstrings"
Text(.Booking.confirmBookingCta)

Xcode derives symbol names by camelCasing the key: settings.notifications.toggle → .settingsNotificationsToggle. You can convert existing extracted strings to symbols via Refactor > Convert Strings to Symbols (reversible).

Generated symbols are internal. For cross-module access, create a public wrapper extension. For heavier multi-module setups, use xcstrings-tool instead.

For the full generated symbols reference — extraction states, symbol derivation rules, and cross-module patterns — see references/string-catalogs.md.

String Types -- Decision Guide

LocalizedStringKey (SwiftUI default)

SwiftUI views accept LocalizedStringKey for their text parameters. String literals are implicitly converted -- no extra work needed.

Text("Welcome back")
Button("Delete") { deleteItem() }

Use LocalizedStringKey when passing strings directly to SwiftUI view initializers. Do not construct LocalizedStringKey manually in most cases.

String(localized:) -- Modern NSLocalizedString replacement

Use for any localized string outside a SwiftUI view initializer. Returns a plain String. The literal/interpolated initializer is available iOS 15+; resolving a LocalizedStringResource is iOS 16+.

let title = String(localized: "Welcome back")
let msg = String(localized: "error.network",
                 defaultValue: "Check your internet connection")

For Swift package localization failures, answer with this explicit resource checklist before bundle debugging:

1. Package.swift declares defaultLocalization.
2. The target resources list processes the catalog location, such as .process("Resources").
3. Localizable.xcstrings is actually inside that processed target-resource path. Only after those pass, debug lookup with bundle: .module or Text(..., bundle: .module).

Existing NSLocalizedString literal keys can still be exported or migrated by Xcode tooling, but new Swift code should prefer String(localized:), SwiftUI literals, LocalizedStringResource, or generated symbols.

LocalizedStringResource -- Pass localization info without resolving

Use when a string must be carried as a localizable value for later resolution, especially for App Intents, widgets, notifications, generated localizable symbols, and system APIs that accept LocalizedStringResource directly. Use String(localized:) when code needs the resolved string immediately. Available iOS 16+.

struct OrderCoffeeIntent: AppIntent {
    static var title: LocalizedStringResource = "Order Coffee"
}

func showAlert(title: LocalizedStringResource, message: LocalizedStringResource) {
    let resolved = String(localized: title)
}

String Interpolation in Localized Strings

Interpolated values in localized strings become positional arguments that translators can reorder.

// English: "Welcome, Alice! You have 3 new messages."
// German:  "Willkommen, Alice! Sie haben 3 neue Nachrichten."
// Japanese: "Alice さん、新しいメッセージが 3 件あります。"
let text = String(localized: "Welcome, \(name)! You have \(count) new messages.")

In the String Catalog, this appears with %@ and %lld placeholders that translators can reorder:

• English: "Welcome, %@! You have %lld new messages."
• Japanese: "%@さん、新しいメッセージが%lld件あります。"

Type-safe interpolation (preferred over format specifiers):

// Interpolation provides type safety
String(localized: "Score: \(score, format: .number)")
String(localized: "Due: \(date, format: .dateTime.month().day())")

Pluralization

String Catalogs handle pluralization natively -- no .stringsdict XML required.

Setup in String Catalog

When a localized string contains an integer interpolation, Xcode detects it and offers plural variants in the String Catalog editor. Supply translations for each CLDR plural category:

English uses only one and other. Arabic uses all six. Always supply other as the fallback.

// Code -- single interpolation triggers plural support
Text("\(unreadCount) unread messages")

// String Catalog entries (English):
//   one:   "%lld unread message"
//   other: "%lld unread messages"

Device Variations

String Catalogs support device-specific text (iPhone vs iPad vs Mac):

// In String Catalog editor, enable "Vary by Device" for a key
// iPhone: "Tap to continue"
// iPad:   "Tap or click to continue"
// Mac:    "Click to continue"

Use Foundation's automatic grammar agreement markup when nearby words must inflect for a value's number or gender. Preserve the complete inflecting phrase for translators; see Automatic Grammar Agreement.

FormatStyle -- Locale-Aware Formatting

Never hard-code user-facing formats. Use FormatStyle and test output under contrasting locales such as en_US, de_DE, ar_SA, and ja_JP.

ios-localization owns FormatStyle guidance when the issue is locale-aware user-facing display, including numbers, dates, currency, units, names, lists, calendars, separators, and locale preview/testing. For custom FormatStyle, ParseableFormatStyle, parsing, Date.IntervalFormatStyle, URL.FormatStyle, or reusable formatter API design, route to swift-formatstyle; keep ios-localization advice to locale risks and testing unless implementation is explicitly requested.

Dates

let now = Date.now

// Preset styles
now.formatted(date: .long, time: .shortened)
// US: "January 15, 2026 at 3:30 PM"
// DE: "15. Januar 2026 um 15:30"
// JP: "2026年1月15日 15:30"

// Component-based
now.formatted(.dateTime.month(.wide).day().year())
// US: "January 15, 2026"

// In SwiftUI
Text(now, format: .dateTime.month().day().year())

Numbers

let count = 1234567
count.formatted()                     // "1,234,567" (US) / "1.234.567" (DE)
count.formatted(.number.precision(.fractionLength(2)))
count.formatted(.percent)             // For 0.85 -> "85%" (US) / "85 %" (FR)

// Currency
let price = Decimal(29.99)
price.formatted(.currency(code: "USD"))  // "$29.99" (US) / "29,99 $US" (FR)
price.formatted(.currency(code: "EUR"))  // "29,99 EUR" (DE)

Measurements

let distance = Measurement(value: 5, unit: UnitLength.kilometers)
distance.formatted(.measurement(width: .wide))
// US: "3.1 miles" (auto-converts!) / DE: "5 Kilometer"

let temp = Measurement(value: 22, unit: UnitTemperature.celsius)
temp.formatted(.measurement(width: .abbreviated))
// US: "72 F" (auto-converts!) / FR: "22 C"

Load references/formatstyle-locale.md for duration, names, lists, custom styles, variant matrices, and deeper RTL testing.

Right-to-Left (RTL) Layout

SwiftUI automatically mirrors layouts for RTL languages (Arabic, Hebrew, Urdu, Persian). Most views require zero changes.

What SwiftUI auto-mirrors

• HStack children reverse order
• .leading / .trailing alignment and padding swap sides
• NavigationStack back button moves to trailing edge
• List disclosure indicators flip
• Text alignment follows reading direction

What needs manual attention

// Testing RTL in previews
MyView()
    .environment(\.layoutDirection, .rightToLeft)
    .environment(\.locale, Locale(identifier: "ar"))

// Images that should mirror (directional arrows, progress indicators)
Image(systemName: "chevron.right")
    .flipsForRightToLeftLayoutDirection(true)

// Images that should NOT mirror: logos, photos, clocks, music notes

// Forced LTR for specific content (phone numbers, code)
Text("+1 (555) 123-4567")
    .environment(\.layoutDirection, .leftToRight)

Layout rules

• DO use .leading / .trailing -- they auto-flip for RTL
• DON'T use .left / .right -- they are fixed and break RTL
• DO use HStack / VStack -- they respect layout direction
• DON'T use absolute offset(x:) for directional positioning

Common Mistakes

DON'T: Use fixed-width layouts

// WRONG -- German text is ~30% longer than English
Text(title).frame(width: 120)

DO: Use flexible layouts

// CORRECT
Text(title).fixedSize(horizontal: false, vertical: true)
// Or use VStack/wrapping that accommodates expansion

DON'T: Skip pseudolocalization testing

Testing only in English hides truncation, layout, and RTL bugs.

DO: Test with German (long) and Arabic (RTL) at minimum

Use Xcode scheme settings to override the app language without changing device locale.

Review Checklist

• All user-facing strings use localization (LocalizedStringKey in SwiftUI or String(localized:))
• No string concatenation for user-visible text
• Dates and numbers use FormatStyle, not hardcoded formats
• Pluralization handled via String Catalog plural variants (not manual if/else)
• Layout uses .leading / .trailing, not .left / .right
• UI tested with long text (German) and RTL (Arabic)
• String Catalog includes all target languages
• Images needing RTL mirroring use .flipsForRightToLeftLayoutDirection(true)
• App Intents and widgets use LocalizedStringResource
• No NSLocalizedString usage in new code
• Comments provided for ambiguous keys (context for translators)
• @ScaledMetric used for spacing that must scale with Dynamic Type
• Currency formatting uses explicit currency code, not locale default
• Pseudolocalization tested (accented, right-to-left, double-length)
• Manually-managed keys use stable symbol-style names, not English text as the key
• Generate String Catalog Symbols enabled for targets with manually-managed keys
• Ensure localized string types are Sendable; use @MainActor for locale-change UI updates

References

• FormatStyle patterns: references/formatstyle-locale.md
• String Catalogs guide: references/string-catalogs.md