This post explains how iOS determines the language to use in an app, and how Apple fallback to the next-best default language when necessary.

Throughout this post, we use the example where en (English) is the developmen and base language, while zh-Hans (Chinese) is an additional supported language.

# en Localizable.strings
"awesome-title" = "Hello World";
"another-title" = "Localization Rocks";
# zh-Hans Localizable.strings
"awesome-title" = "你好";

Deliberately, the zh-Hans strings file is 50% localized (another-title is not translated).

How Apple determines the langauge

Apple explains the process. Here is the “alogrithm” in pseudocode:

func determineTheLanguageToUse():
  for each user's preferredLanguages
    if app supports the language
      return the language
    if app supports a more generic dialect
      return the generic language

  # Exhausted preferredLanguages and still cannot determine..
  return CFBundleDevelopmentRegion

User’s preferredLanguages is those listed in Settings App > General > Language & Region.

The algo starts with the most preferred language, checks if the app supports it (or a more generic dialect), before finally using CFBundleDevelopmentRegion.

There are different cases of fallback. Let’s look at them in detail.

Fallback 1: Generic Dialect

In the algo, iOS will check if there is a more generic dialect for the preferred language, and if so return that.

What is a more generic dialect? en is more generic than en-GB (British English). In our example, if user prefers en-GB, then en will be used.

This is because the app does NOT have en-GB, but en is good enough.

The other way round is not true. If the app supports en-GB (and not en), then if user prefers en, then en-GB will not be the fallback – because en-GB is not more generic.

Fallback 2: Unsupported Language

An unsupported language is when all preferredLanguages is exhausted, and the app does not have a suitable language to use.

For example if a user prefers ms (Malay), but which the app does not support at all, then the language specified in CFBundleDevelopmentRegion of the Info.plist will be used. This is aka the localization native development region.

This is a very common case. When we lanuch an app, we probably support only a few languages, or just one!

Fallback 3: Unsupported Phrase

This is an obscure case, and is not mention in Apple’s documentation, nor in the alogrithm.

Let’s take the example where a user prefers zh-Hans.

What happens when iOS try to get the NSLocalizedString of “another-title”?

Remember, zh-Hans is not fully translated. It does not have “another-title”. What do you think will happen?

  1. Fallback to development language en or
  2. Return “another-title”

Many developers think it is (1). Unfortunately, it is (2), with that ugly key name!

If you refer to the determineTheLanguageToUse algo, the language to use is still zh-Hans, regardless that it is incomplete. It can’t find “another-title”, so it just return the key as value..

iOS should really improve on this fallback behaviour.. for now, we need some custom code.

The Fallback Code

LS is a global function to replace NSLocalizedString. It is a shorthand, with added fallback capability:

public func LS(_ key: String) -> String {
    let value = NSLocalizedString(key, comment: "")
    if value != key || NSLocale.preferredLanguages.first == "en" {
        return value

    // Fall back to en
        let path = Bundle.main.path(forResource: "en", ofType: "lproj"),
        let bundle = Bundle(path: path)
        else { return value }
    return NSLocalizedString(key, bundle: bundle, comment: "")

When a phrase is not yet translated, it will be value == key, which is dumb, so we fall back to using en.

The rest of the code is simply geting NSLocalizedString from the en bundle.

More Resourses

Apple’s Technical Note on language identifiers




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