Mobile — Localization (i18n / l10n)

Mobile localization is mostly per-platform: each store has its own metadata locales, and each platform has its own string format. Cross-platform stacks (RN, Flutter, KMP) ship their own i18n libraries. For web i18n see i18n.

iOS

• ★ String Catalogs (.xcstrings, Xcode 15+) — Apple's modern format; replaces .strings and .stringsdict. Plurals, gender, varied formats, automatic extraction. The default for new projects.
• .strings / .stringsdict — older format; still works.
• NSLocalizedString — runtime API; Xcode auto-extracts on build.
• String Catalog Editor — built into Xcode.

Android

• ★ strings.xml + plurals.xml — XML resources per locale.
• ICU MessageFormat — Android supports ICU plurals/select via <plurals> and formatted strings.
• Compose stringResource(R.string.X) — runtime resolver.
• Per-app language preferences (Android 13+) — let users pick a language inside your app.

Cross-platform

React Native

• ★ i18next + react-i18next — most popular.
• react-intl (FormatJS) — ICU-style.
• Lingui (@lingui/core + @lingui/react) — modern type-safe.
• react-native-localize — read user/device locale settings.

Flutter

• ★ intl package + ARB files — official.
• easy_localization — simpler runtime API.
• slang — type-safe code-gen.

KMP / Compose Multiplatform

• moko-resources — multiplatform string/font/image resources.
• compose-multiplatform-resources — official, replacing moko in many projects.
• lyricist — Compose-friendly i18n.

Translation management systems (TMS)

• ★ Crowdin — popular for mobile; integrates with App Store Connect / Play Console; free tier for OSS.
• Lokalise — paid; clean API + developer DX.
• POEditor — generous free tier; simple UI.
• Phrase (formerly PhraseApp) — paid; enterprise.
• Transifex — paid.
• ★ Tolgee — open-source, self-hostable; free hosted tier; in-context editor browser extension. Apache 2.0.
• Lokalize (mobile-flavored Phrase product).
• Languine — newer; AI-assisted; see i18n.
• Weblate — open-source; popular for FOSS projects.

Workflow patterns

• Source → TMS → translators → PR: Crowdin / Tolgee / Lokalise auto-PR translated files back into your repo.
• AI pre-translation + human review: Crowdin and Lokalise both offer this; cuts translator time.
• Continuous localization: TMS picks up new strings on every push, translators see them within minutes.

Store metadata localization

• App Store Connect supports ~40 locales; Google Play Console similar; treat metadata as strings managed in your TMS.
• fastlane deliver / supply can push localized metadata.

RTL / bi-directional

• iOS: SwiftUI handles RTL automatically; test Arabic/Hebrew layouts.
• Android: android:supportsRtl="true" in manifest; Compose handles automatically.
• Always snapshot-test RTL — see Mobile — Snapshot Testing.

Pluralization / formatted numbers

• ICU MessageFormat is the right shape for plurals.
• String Catalog uses Apple-flavored plurals.
• Android plurals.xml uses CLDR plural categories.
• Don't concatenate strings — let the platform format dates / numbers / currency.

Pick this if…

• iOS-only: String Catalogs natively; pair with Crowdin or Tolgee for translators.
• Android-only: strings.xml + plurals; same TMS for translators.
• RN: i18next or Lingui + Crowdin.
• Flutter: intl + ARB + Crowdin or slang for type safety.
• KMP shared resources: compose-multiplatform-resources or moko-resources.
• OSS / self-host TMS: Tolgee or Weblate.