Internationalization (i18n)

Angular's internationalization (i18n) tools help make your app available in multiple languages.

Table of contents

• Angular and i18n template translation
• Mark text with the i18n attribute
• Add i18n-... translation attributes
• Handle singular and plural
• Select among alternative texts
• Create a translation source file with the ng-xi18n extraction tool
• Translate text messages
• Merge the completed translation file into the app
  • Merge with the JIT compiler
  • Internationalization with the AOT compiler
• Translation file maintenance and id changes

Try this live example of a JIT-compiled app, translated into Spanish.

Angular and i18n template translation

Application internationalization is a challenging, many-faceted effort that takes dedication and enduring commitment. Angular's i18n internationalization facilities can help.

This page describes the i18n tools available to assist translation of component template text into multiple languages.

The i18n template translation process has four phases:

1. Mark static text messages in your component templates for translation.

2. An angular i18n tool extracts the marked messages into an industry standard translation source file.

3. A translator edits that file, translating the extracted text messages into the target language, and returns the file to you.

4. The Angular compiler imports the completed translation files, replaces the original messages with translated text, and generates a new version of the application in the target language.

You need to build and deploy a separate version of the application for each supported language.

Mark text with the i18n attribute

The Angular i18n attribute is a marker for translatable content. Place it on every element tag whose fixed text should be translated.

In the accompanying sample, an <h1> tag displays a simple English language greeting that you translate into Spanish:

Add the i18n attribute to the tag to mark it for translation.

Help the translator with a description and intent

In order to translate it accurately, the translator may need a description of the message. Assign a description to the i18n attribute:

In order to deliver a correct translation, the translator may need to know your intent—the true meaning of the text within this particular application context. In front of the description, add some contextual meaning to the assigned string, separating it from the description with the | character (<meaning>|<description>):

While all appearances of a message with the same meaning have the same translation, a message with a variety of possible meanings could have different translations. The Angular extraction tool preserves both the meaning and the description in the translation source file to facilitiate contextually-specific translations.

Translate text without creating an element

Suppose there is a stretch of text that you'd like to translate. You could wrap it in a <span> tag but for some reason (CSS comes to mind) you don't want to create a new DOM element merely to facilitate translation.

Here are two techniques to try.

(1) Wrap the text in an <ng-container> element. The <ng-container> is never renderered:

(2) Wrap the text in a pair of HTML comments:

Add i18n-... translation attributes

You've added an image to your template. You care about accessibility too so you add a title attribute:

The title attribute needs to be translated. Angular i18n support has more translation attributes in the form,i18n-x, where x is the name of the attribute to translate.

To translate the title on the img tag from the previous example, write:

You can also assign a meaning and a description with the i18n-x="<meaning>|<description>" syntax.

Handle singular and plural

Different languages have different pluralization rules.

Suppose your application says something about a collection of wolves. In English, depending upon the number of wolves, you could display "no wolves", "one wolf", "two wolves", or "a wolf pack". Other languages might express the cardinality differently.

Here's how you could mark up the component template to display the phrase appropriate to the number of wolves:

• The first parameter is the key. It is bound to the component property (wolves) that determines the number of wolves.
• The second parameter identifies this as a plural translation type.
• The third parameter defines a pluralization pattern consisting of pluralization categories and their matching values.

Pluralization categories include:

• =0
• =1
• =5
• few
• other

Put the default English translation in braces ({}) next to the pluralization category.

• When you're talking about one wolf, you could write =1 {one wolf}.
• For zero wolves, you could write =0 {no wolves}.
• For two wolves, you could write =2 {two wolves}.

You could keep this up for three, four, and every other number of wolves. Or you could specify the other category as a catch-all for any unmatched cardinality and write something like: other {a wolf pack}.

Select among alternative texts

The application displays different text depending upon whether the hero is male or female. These text alternatives require translation too.

You can handle this with a select translation. A select also follows the ICU message syntax. You choose among alternative translation based on a string value instead of a number.

The following format message in the component template binds to the component's gender property, which outputs either an "m" or an "f". The message maps those values to the appropriate translation:

Create a translation source file with the ng-xi18n tool

Use the ng-xi18n extraction tool to extract the i18n-marked texts into a translation source file in an industry standard format.

This is an Angular CLI tool in the @angular/compiler-cli npm package. If you haven't already installed the CLI and its platform-server peer dependency, do so now:

Open a terminal window at the root of the application project and enter the ng-xi18n command:

By default, the tool generates a translation file named messages.xlf in the XML Localisation Interchange File Format (XLIFF, version 1.2).

Other translation formats

You can generate a file named messages.xmb in the XML Message Bundle (XMB) format by adding the --i18nFormat=xmb flag.

This sample sticks with the XLIFF format.

Other options

You may have to specify additional options. For example, if the tsconfig.json TypeScript configuration file is located somewhere other than in the root folder, you must identify the path to it with the -p option:

Add an npm script for convenience

Consider adding a convenience shortcut to the scripts section of the package.json to make the command easier to remember and run:

Now you can issue command variations such as these:

Note the -- flag before the options. It tells npm to pass every flag thereafter to ng-xi18n.

Translate text messages

The ng-xi18n command generates a translation source file in the project root folder named messages.xlf. The next step is to translate the English language template text into the specific language translation files. The cookbook sample creates a Spanish translation file.

Create a localization folder

You will probably translate into more than one other language so it's a good idea for the project structure to reflect your entire internationalization effort.

One approach is to dedicate a folder to localization and store related assets (for example, internationalization files) there.

This cookbook follows that suggestion. It has a locale folder under the src/. Assets within the folder carry a filename extension that matches a language-culture code from a well-known codeset.

Make a copy of the messages.xlf file, put it in the locale folder and rename it messages.es.xlffor the Spanish language translation. Do the same for each target language.

Translate text nodes

In the real world, you send the messages.es.xlf file to a Spanish translator who fills in the translations using one of the many XLIFF file editors.

This sample file is easy to translate without a special editor or knowledge of Spanish. Open messages.es.xlf and find the first <trans-unit> section:

This XML element represents the translation of the <h1> greeting tag you marked with the i18n attribute.

Using the source, description, and meaning elements to guide your translation, replace the <target/> tag with the Spanish greeting:

Translate the other text nodes the same way:

Translate plural and select

Translating plural and select messages is a little tricky.

The <source> tag is empty for plural and select translation units, which makes them hard to correlate with the original template. The XLIFF format doesn't yet support the ICU rules; it soon will. However, the XMB format does support the ICU rules.

You'll just have to look for them in relation to other translation units that you recognize from elsewhere in the source template. In this example, you know the translation unit for the select must be just below the translation unit for the logo.

Translate plural

To translate a plural, translate its ICU format match values:

Translate select

The select behaves a little differently. Here again is the ICU format message in the component template:

The extraction tool broke that into two translation units.

The first unit contains the text that was outside the select. In place of the select is a placeholder, <x id="ICU">, that represents the select message. Translate the text and leave the placeholder where it is.

The second translation unit, immediately below the first one, contains the select message. Translate that.

Here they are together, after translation:

The entire template translation is complete. It's time to incorporate that translation into the application.

The app before translation

When the previous steps finish, the sample app and its translation file are as follows:

Merge the completed translation file into the app

To merge the translated text into component templates, compile the application with the completed translation file. The process is the same whether the file is in .xlf format or in another format (.xlif and .xtb) that Angular understands.

You provide the Angular compiler with three new pieces of information:

• the translation file
• the translation file format
• the Locale ID (es or en-US for instance)

How you provide this information depends upon whether you compile with the JIT (Just-in-Time) compiler or the AOT (Ahead-of-Time) compiler.

• With JIT, you provide the information at bootstrap time.
• With AOT, you pass the information as ngc options.

Merge with the JIT compiler

The JIT compiler compiles the application in the browser as the application loads. Translation with the JIT compiler is a dynamic process of:

1. Determining the language version for the current user.
2. Importing the appropriate language translation file as a string constant.
3. Creating corresponding translation providers to guide the JIT compiler.
4. Bootstrapping the application with those providers.

Open index.html and revise the launch script as follows:

In this sample, the user's language is hardcoded as a global document.locale variable in the index.html.

SystemJS Text plugin

Notice the SystemJS mapping of text to a systemjs-text-plugin.js. With the help of a text plugin, SystemJS can read any file as raw text and return the contents as a string. You'll need it to import the language translation file.

SystemJS doesn't ship with a raw text plugin but it's easy to add. Create the following systemjs-text-plugin.js in the src/ folder:

Create translation providers

Three providers tell the JIT compiler how to translate the template texts for a particular language while compiling the application:

• TRANSLATIONS is a string containing the content of the translation file.
• TRANSLATIONS_FORMAT is the format of the file: xlf, xlif or xtb.
• LOCALE_ID is the locale of the target language.

The getTranslationProviders function in the following src/app/i18n-providers.ts creates those providers based on the user's locale and the corresponding translation file:

1. It gets the locale from the global document.locale variable that was set in index.html.

2. If there is no locale or the language is U.S. English (en-US), there is no need to translate. The function returns an empty noProviders array as a Promise. It must return a Promise because this function could read a translation file asynchronously from the server.

3. It creates a transaction filename from the locale according to the name and location convention described earlier.

4. The getTranslationsWithSystemJs method reads the translation and returns the contents as a string. Notice that it appends !text to the filename, telling SystemJS to use the text plugin.

5. The callback composes a providers array with the three translation providers.

6. Finally, getTranslationProviders returns the entire effort as a promise.

Bootstrap the app with translation providers

The Angular bootstrapModule method has a second, options parameter that can influence the behavior of the compiler.

You'll create an options object with the translation providers from getTranslationProviders and pass it to bootstrapModule. Open the src/main.ts and modify the bootstrap code as follows:

Notice that it waits for the getTranslationProviders promise to resolve before bootstrapping the app.

The app is now internationalized for English and Spanish and there is a clear path for adding more languages.

Internationalize with the AOT compiler

The JIT compiler translates the application into the target language while compiling dynamically in the browser. That's flexible but may not be fast enough for your users.

The AOT (Ahead-of-Time) compiler is part of a build process that produces a small, fast, ready-to-run application package. When you internationalize with the AOT compiler, you pre-build a separate application package for each language. Then in the host web page (index.html), you determine which language the user needs and serve the appropriate application package.

This cookbook doesn't cover how to build multiple application packages and serve them according to the user's language preference. It does explain the few steps necessary to tell the AOT compiler to apply a translations file.

Internationalization with the AOT compiler requires some setup specifically for AOT compilation. Start with the application project as shown just before merging the translation file and refer to the AOT cookbook to make it AOT-ready.

Next, issue an ngc compile command for each supported language (including English). The result is a separate version of the application for each language.

Tell AOT how to translate by adding three options to the ngc command:

• --i18nFile: the path to the translation file
• --locale: the name of the locale
• --i18nFormat: the format of the localization file

For this sample, the Spanish language command would be

Translation file maintenance and id changes

As the application evolves, you will change the i18n markup and re-run the ng-xi18n extraction tool many times. The new markup that you add is not a problem; but most changes to existing markup trigger generation of new ids for the affected translation units.

After an id changes, the translation files are no longer in-sync. All translated versions of the application will fail during re-compilation. The error messages identify the old ids that are no longer valid but they don't tell you what the new ids should be.

Commit all translation message files to source control, especially the English source messages.xlf. The difference between the old and the new messages.xlf file help you find and update id changes across your translation files.