iOS Localization & Internationalization

Localize Apple-platform applications using String Catalogs (.xcstrings), compiler-generated symbols, locale-aware FormatStyle, and right-to-left (RTL) layout adaptation. Targets Swift 6.4 / iOS 26+.

String Catalogs (.xcstrings)
String Types: Key vs Resource
Pluralization & Variations
FormatStyle & Locales
Right-to-Left (RTL) Layout
iOS 27 Updates
Common Mistakes
Review Checklist
References

String Catalogs (.xcstrings)

String Catalogs are the standard format for Xcode localization. Xcode automatically extracts localizable strings from source code on every build:

// Automatically extracted as LocalizedStringKey in SwiftUI
Text("Welcome back")
Button("Save Changes") { }
Label("Account Settings", systemImage: "person.crop.circle")

For non-view code, use generated symbols or String(localized:):

// Modern type-safe localized string
let message = String(localized: "order_confirmation_title", defaultValue: "Order Confirmed")

String Types: Key vs Resource

LocalizedStringKey: Used across SwiftUI views (Text, Button). Evaluated lazily at view render time.
LocalizedStringResource: Decoupled from SwiftUI. Ideal for passing localized strings across view models, models, or App Intents.
String(localized:): Immediately materializes the localized String using the current locale.

Pluralization & Variations

String Catalogs support Unicode plural rules (zero, one, two, few, many, other) and device-specific variations (iPhone, iPad, Mac) in the visual catalog editor without complex .stringsdict files:

// Single integer interpolation triggers plural variant support in String Catalogs
Text("\(unreadCount) unread messages")

FormatStyle & Locales

Never format currency, numbers, or dates with manual string concatenation. Always use FormatStyle:

// Currency formatting adhering to user locale
let priceString = amount.formatted(.currency(code: "USD"))

// Date and relative time formatting
let dateString = eventDate.formatted(date: .abbreviated, time: .shortened)
let relativeString = eventDate.formatted(.relative(presentation: .named))

Right-to-Left (RTL) Layout

Use leading/trailing layout constraints rather than absolute left/right.
Directional SF Symbols mirror automatically; set .imageScale or disable mirroring only when representing physical hardware orientation.

iOS 27 Updates

Xcode 27 agents can translate String Catalogs; the editor adds a Generate Translations button. Review generated translations like any machine translation.
Strings with a "do not translate" comment are marked accordingly. Export extracts localization macros from header files.
Background Assets supports localized asset packs matching the user's preferred languages.

Sources: https://sosumi.ai/documentation/xcode-release-notes/xcode-27-release-notes, https://sosumi.ai/documentation/ios-ipados-release-notes/ios-ipados-27-release-notes

Common Mistakes

String concatenation for localized sentences: Concatenating strings breaks syntax across languages. Use format specifiers with positional arguments (%1$@).
Hardcoding date or number formatting: Manual decimal or comma placement breaks in non-US locales. Use FormatStyle.
Using left/right alignment: Hardcoded left/right alignments do not flip in Arabic/Hebrew RTL layouts. Use leading/trailing.
Eager localization in static constants: Initializing String(localized:) in static properties pins the language at launch, failing when the user switches language dynamically.
Ignoring pluralization variations: Hardcoding s for counts fails across languages with complex plural rules (e.g. Russian, Arabic).

Review Checklist

String Catalogs (.xcstrings) used for all new string localization
LocalizedStringResource used when passing localized strings from model layers
Numeric, date, and currency values formatted via Foundation FormatStyle
Layout constraints and padding use leading and trailing
Inflection and plural rules configured in String Catalogs

References

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