IOS Localization

iOS Localization & Internationalization

Localize iOS 26+ apps using String Catalogs, modern string types, FormatStyle, and RTL-aware layout. Localization mistakes cause App Store rejections in non-English markets, mistranslated UI, and broken layouts. Ship with correct localization from the start.

Contents

• String Catalogs (.xcstrings)
• 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 (.xcstrings)

String Catalogs replaced .strings and .stringsdict files starting in Xcode 15 / iOS 17. They unify all localizable strings, pluralization rules, and device variations into a single JSON-based file with a visual editor.

Why String Catalogs exist:

• .strings files required manual key management and fell out of sync
• .stringsdict required complex XML for plurals
• String Catalogs auto-extract strings from code, track translation state, and support plurals natively

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")

// NOT extracted -- plain String, not localized
let msg = "Hello"                 // just a String, invisible to Xcode

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.

String Types -- Decision Guide

LocalizedStringKey (SwiftUI default)

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

// These all create a LocalizedStringKey lookup automatically:
Text("Welcome back")
Label("Profile", systemImage: "person")
Button("Delete") { deleteItem() }
NavigationTitle("Home")

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. Available iOS 16+.

// Basic
let title = String(localized: "Welcome back")

// With default value (key differs from English text)
let msg = String(localized: "error.network",
                 defaultValue: "Check your internet connection")

// With table and bundle
let label = String(localized: "onboarding.title",
                   table: "Onboarding",
                   bundle: .module)

// With comment for translators
let btn = String(localized: "Save",
                 comment: "Button title to save the current document")

LocalizedStringResource -- Pass localization info without resolving

Use when you need to pass a localized string to an API that resolves it later (App Intents, widgets, notifications, system frameworks). Available iOS 16+.

// App Intents require LocalizedStringResource
struct OrderCoffeeIntent: AppIntent {
    static var title: LocalizedStringResource = "Order Coffee"
}

// Widgets
struct MyWidget: Widget {
    var body: some WidgetConfiguration {
        StaticConfiguration(kind: "timer",
                            provider: Provider()) { entry in
            TimerView(entry: entry)
        }
        .configurationDisplayName(LocalizedStringResource("Timer"))
    }
}

// Pass around without resolving yet
func showAlert(title: LocalizedStringResource, message: LocalizedStringResource) {
    // Resolved at display time with the user's current locale
    let resolved = String(localized: title)
}

When to use each type

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"

Grammar Agreement (iOS 17+)

Use ^[...] inflection syntax for automatic grammatical agreement:

// Automatically adjusts for gender/number in supported languages
Text("^[\(count) \("photo")](inflect: true) added")
// English: "1 photo added" / "3 photos added"
// Spanish: "1 foto agregada" / "3 fotos agregadas"

FormatStyle -- Locale-Aware Formatting

Never hard-code date, number, or measurement formats. Use FormatStyle (iOS 15+) so formatting adapts to the user's locale automatically.

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"

Duration, PersonName, Lists

// Duration
let dur = Duration.seconds(3661)
dur.formatted(.time(pattern: .hourMinuteSecond))  // "1:01:01"

// Person names
let name = PersonNameComponents(givenName: "John", familyName: "Doe")
name.formatted(.name(style: .long))   // "John Doe" (US) / "Doe John" (JP)

// Lists
let items = ["Apples", "Oranges", "Bananas"]
items.formatted(.list(type: .and))    // "Apples, Oranges, and Bananas" (EN)
                                      // "Apples, Oranges et Bananas" (FR)

For the complete FormatStyle reference, custom styles, and RTL layout, see references/formatstyle-locale.md.

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 NSLocalizedString in new code

// WRONG -- legacy API, verbose, no compiler integration with String Catalogs
let title = NSLocalizedString("welcome_title", comment: "Welcome screen title")

DO: Use String(localized:) or let SwiftUI handle it

// CORRECT
let title = String(localized: "welcome_title",
                   defaultValue: "Welcome!",
                   comment: "Welcome screen title")
// Or in SwiftUI, just:
Text("Welcome!")

DON'T: Concatenate localized strings

// WRONG -- word order varies by language
let greeting = String(localized: "Hello") + ", " + name + "!"

DO: Use string interpolation

// CORRECT -- translators can reorder placeholders
let greeting = String(localized: "Hello, \(name)!")

DON'T: Hard-code date/number formats

// WRONG -- US-only format
let formatter = DateFormatter()
formatter.dateFormat = "MM/dd/yyyy"  // Meaningless in most countries

DO: Use FormatStyle

// CORRECT -- adapts to user locale
Text(date, format: .dateTime.month().day().year())

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: Use .left / .right for alignment

// WRONG -- does not flip for RTL
HStack { Spacer(); text }.padding(.left, 16)

DO: Use .leading / .trailing

// CORRECT
HStack { Spacer(); text }.padding(.leading, 16)

DON'T: Put user-facing strings as plain String outside SwiftUI

// WRONG -- not localized
let errorMessage = "Something went wrong"
showAlert(message: errorMessage)

DO: Use LocalizedStringResource for deferred resolution

// CORRECT
let errorMessage = LocalizedStringResource("Something went wrong")
showAlert(message: String(localized: errorMessage))

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)
• 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