The Complete Guide to iOS App Localization

// In Xcode:
// 1. Select your project in the navigator
// 2. Go to Info tab > Localizations
// 3. Click + to add languages (e.g., German, Japanese)
// 4. Select which files to localize
//
// Xcode creates .lproj directories automatically:
// en.lproj/Localizable.strings
// de.lproj/Localizable.strings
// ja.lproj/Localizable.strings

// Base.lproj/Localizable.strings

"welcome_title" = "Welcome to MyApp";
"login_button" = "Sign In";
"settings_label" = "Settings";
"greeting" = "Hello, %@!";    // %@ = string placeholder
"item_count" = "%d items";     // %d = integer placeholder

// ❌ Common mistakes in .strings files:

// Missing semicolon — file loads but translations are empty
"welcome_title" = "Welcome"

// Unescaped quotes — causes parse error
"message" = "Click "here" to continue";

// ✅ Correct versions:
"welcome_title" = "Welcome";
"message" = "Click \"here\" to continue";

// Xcode 15+ String Catalog (Localizable.xcstrings)
// Xcode automatically extracts strings from your code
// and manages translations in a visual editor.

// In SwiftUI, strings are automatically localizable:
Text("Welcome to MyApp")
Text("Hello, \(userName)!")

// Mark strings explicitly:
let title = String(localized: "welcome_title")

import SwiftUI

struct ContentView: View {
    let userName: String

    var body: some View {
        VStack {
            // ✅ SwiftUI auto-localizes string literals
            Text("welcome_title")

            // ⚠️ This does NOT localize (String interpolation)
            // Text("Hello, \(userName)")

            // ✅ Use String(localized:) for dynamic strings
            Text(String(localized: "greeting \(userName)"))

            // ✅ UIKit style (works everywhere)
            let title = NSLocalizedString(
                "settings_label",
                comment: "Settings screen title"
            )

            // ✅ Modern API (iOS 16+)
            let modern = String(
                localized: "welcome_title",
                comment: "Main screen title"
            )
        }
    }
}

<!-- Localizable.stringsdict -->
<?xml version="1.0" encoding="UTF-8"?>
<plist version="1.0">
<dict>
    <key>items_count</key>
    <dict>
        <key>NSStringLocalizedFormatKey</key>
        <string>%#@count@</string>
        <key>count</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 in Swift:
String(format: NSLocalizedString("items_count", comment: ""),
       itemCount)

# Install Fastlane
$ gem install fastlane

# Initialize deliver for App Store metadata
$ fastlane deliver init

# Directory structure created:
# fastlane/metadata/
# ├── en-US/
# │   ├── name.txt            # App name (30 chars)
# │   ├── subtitle.txt        # Subtitle (30 chars)
# │   ├── description.txt     # Full description
# │   ├── keywords.txt        # Search keywords (100 chars)
# │   ├── release_notes.txt   # What's New
# │   └── promotional_text.txt
# ├── de-DE/
# │   └── ...
# └── ja/
#     └── ...

# Push metadata to App Store Connect:
$ fastlane deliver

// 1. Xcode Scheme Override:
// Edit Scheme > Run > Options > App Language > Choose language

// 2. SwiftUI Preview with locale:
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
            .environment(\.locale, Locale(identifier: "de"))

        ContentView()
            .environment(\.locale, Locale(identifier: "ja"))

        ContentView()
            .environment(\.locale, Locale(identifier: "ar"))
    }
}

// 3. XCUITest with language override:
let app = XCUIApplication()
app.launchArguments += ["-AppleLanguages", "(de)"]
app.launchArguments += ["-AppleLocale", "de_DE"]
app.launch()

# Translate .strings files
> Translate Base.lproj/Localizable.strings
  to Japanese, German, and Spanish

# Translate App Store metadata too
> Translate fastlane/metadata/en-US/
  to de-DE, ja, es-MX

✓ 6 files translated in 3.2s

MyApp/
├── MyApp.xcodeproj
├── MyApp/
│   ├── Base.lproj/
│   │   ├── Localizable.strings       # Source strings
│   │   └── Localizable.stringsdict   # Plural rules
│   ├── en.lproj/
│   │   └── Localizable.strings
│   ├── de.lproj/
│   │   └── Localizable.strings
│   ├── ja.lproj/
│   │   └── Localizable.strings
│   ├── Localizable.xcstrings          # OR String Catalog
│   └── Info.plist
├── MyAppTests/
├── fastlane/
│   ├── Fastfile
│   └── metadata/
│       ├── en-US/
│       │   ├── name.txt
│       │   ├── description.txt
│       │   └── keywords.txt
│       ├── de-DE/
│       └── ja/
└── Package.swift