How do you localise strings: Localizable.strings, string catalogs, String(localized:)?

2.6.3 · L1 · Interview

Predict first

What does this print when no translation exists?

print(String(localized: "Hello"))
  • Hello
  • nil
  • An empty string

Mental model. A localisation key is a coat-check ticket. Hand it in with the language and you get the right coat back; lose the coat and you get the ticket text.

You mark user-visible text with a lookup instead of a literal: String(localized: "Hello") (iOS 15), NSLocalizedString, or in SwiftUI a string literal passed to Text, which is a LocalizedStringKey. Translations live in Localizable.strings files per language, or in a single string catalog (.xcstrings, Xcode 15) that Xcode fills in from your code and tracks per language.

If a key has no translation, the lookup returns the key itself, which is why a missing translation shows English-looking text rather than failing. LocalizedStringResource defers the lookup so a value can be passed around and resolved later in the right language. Give translators context with a comment:, use the table: and bundle: parameters for packages, and do not translate strings that are identifiers or logs.

A missing key falls back to the key text itself. That is why untranslated UI still looks fine, and why missing translations are easy to ship.

Common traps

  • Building sentences by concatenation. Word order differs by language, so fragments cannot be translated. Translate whole sentences.
  • Using Text(verbatim:) where you want translation, or the reverse. Verbatim skips the lookup on purpose.

They might follow up with

  • What is a good key? Either the full source-language text (natural keys) or a stable identifier like login.title, applied consistently.
  • How do you test translations? Run with pseudolanguages and another Application Language in the scheme.

Related questions

  • 2.6.4 How do you handle plurals and word order in localised strings?
  • 2.6.5 How does the app pick its language at runtime, and how is that different from Locale?
  • 2.6.2 Why does Bundle.main fail in packages, extensions and tests, and what do you use instead?

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