Supporting Internationalization and Localization

Supporting Internationalization and Localization

Internationalization (i18n) is the process of designing an application so it can be adapted to different languages and regions. Localization (l10n) is the process of actually adapting it for a specific locale. Java's Locale class is the cornerstone of both.

What Is a Locale?

A Locale object represents a specific geographical, political, or cultural region. It encodes:

  • Language — a lowercase ISO 639 code (e.g., "en", "fr", "pt")
  • Country / Region — an uppercase ISO 3166 code (e.g., "US", "FR", "BR")
  • Variant — optional, for further distinctions

A locale does not translate text by itself; it tells locale-sensitive classes how to format or parse values for that region.

Creating Locales

There are several ways to create a Locale:

Using Built-in Constants

java
Locale us      = Locale.US;          // en_US
Locale france  = Locale.FRANCE;      // fr_FR
Locale germany = Locale.GERMANY;     // de_DE
Locale uk      = Locale.UK;          // en_GB

Using the Constructor

java
Locale ptBR = new Locale("pt", "BR");   // Portuguese, Brazil
Locale es    = new Locale("es");         // Spanish (no country)

Using Locale.of() (Java 19+, preferred on newer exams)

java
Locale ptPT = Locale.of("pt", "PT");   // Portuguese, Portugal

Using Locale.Builder

java
Locale custom = new Locale.Builder()
        .setLanguage("ja")
        .setRegion("JP")
        .build();

Getting the Default Locale

The JVM picks up the host system locale on startup:

java
Locale defaultLocale = Locale.getDefault();
System.out.println(defaultLocale);           // e.g., en_US
System.out.println(defaultLocale.getLanguage());  // en
System.out.println(defaultLocale.getCountry());   // US
System.out.println(defaultLocale.getDisplayName()); // English (United States)

You can also change the default programmatically (affects the entire JVM):

java
Locale.setDefault(Locale.FRANCE);

Locale-Sensitive Classes

Many standard library classes accept a Locale to alter their behavior:

Class Locale effect
NumberFormat Digit grouping separator, decimal separator, currency symbol
DateTimeFormatter Month names, day names, date ordering
Collator Language-specific string sorting
String.toLowerCase(Locale) / toUpperCase(Locale) Correct case conversion for locale (e.g., Turkish i → İ)
ResourceBundle Loads locale-specific property files 
java
import java.text.NumberFormat;
import java.util.Locale;

double amount = 1234.56;

NumberFormat us = NumberFormat.getCurrencyInstance(Locale.US);
NumberFormat de = NumberFormat.getCurrencyInstance(Locale.GERMANY);
NumberFormat jp = NumberFormat.getCurrencyInstance(Locale.JAPAN);

System.out.println(us.format(amount));   // $1,234.56
System.out.println(de.format(amount));   // 1.234,56 €
System.out.println(jp.format(amount));   // ¥1,235

Internationalization vs. Localization

Term Abbreviation Definition
Internationalization i18n (18 letters between i and n) Designing/coding the app to be locale-neutral
Localization l10n (10 letters between l and n) Providing locale-specific data (strings, formats)

Practical checklist for i18n-ready code:

  • Never hard-code user-facing strings — use resource bundles.
  • Never hard-code date or number formats — use DateTimeFormatter and NumberFormat with a Locale.
  • Use Locale-aware string comparison and case conversion.
  • Store dates internally as UTC; convert to local time for display.

Locale Display Methods

java
Locale locale = Locale.FRANCE;

System.out.println(locale.getLanguage());           // fr
System.out.println(locale.getCountry());            // FR
System.out.println(locale.getDisplayLanguage());    // French  (in default locale)
System.out.println(locale.getDisplayCountry());     // France
System.out.println(locale.getDisplayName());        // French (France)
System.out.println(locale.toString());              // fr_FR
System.out.println(locale.toLanguageTag());         // fr-FR  (IETF BCP 47)

toLanguageTag() uses hyphens (BCP 47 standard); toString() uses underscores (Java legacy format).

Key Rules Summary

  • Locale encodes language + optional country + optional variant.
  • Use constants (Locale.US, Locale.FRANCE) when available; use Locale.of() or the constructor for others.
  • Locale.getDefault() returns the JVM's current default locale, which mirrors the host OS.
  • Locale does not translate text — it guides formatting classes and resource bundle selection.
  • Always pass a Locale to locale-sensitive APIs rather than relying on the default, so behavior is predictable across environments.

References