AppLocalizeBehavior wraps the format.js library to help you internationalize your application. Note that if you're on a browser that does not natively support the Intl object, you must load a polyfill yourself. An example polyfill can be found at github.com/andyearnshaw/Intl.js.

AppLocalizeBehavior supports the same message syntax as format.js, in its entirety; use the library docs as reference for the available message formats and options.

npm install --save @polymer/app-localize-behavior@next

AppLocalizeBehavior is a legacy behavior. To use it in Polymer 3.0, you need the utility class mixinBehaviors as well:

Import AppLocalizeBehavior

// Import the PolymerElement base class
import {PolymerElement, html} from '@polymer/polymer/polymer-element.js';

// Import AppLocalizeBehavior 
import {AppLocalizeBehavior} from '@polymer/app-localize-behavior/app-localize-behavior.js';

// Import a utility class to treat legacy behaviors as mixins
import {mixinBehaviors} from '@polymer/polymer/lib/legacy/class.js';

// Define your class, inheriting AppLocalizeBehavior functions and properties
class MyApp extends mixinBehaviors([AppLocalizeBehavior], PolymerElement) {...}

The main application is usually responsible for loading the localized messages and setting the current language.

my-app.js

// Imports go here
class MyApp extends mixinBehaviors([AppLocalizeBehavior], PolymerElement) {
  static get template(){
    return html`
      <div>{{localize('hello', 'name', 'Batman')}}</div>
    `;
  }
  constructor() { 
    super();
    // set default language and localization strings
    this.language = 'en';
    this.resources = {
      'en': { 'hello': 'My name is {name}.' },
      'fr': { 'hello': 'Je m\'apelle {name}.' }
    };
  }
}
customElements.define('my-app', MyApp);

index.html

<head>
  <script src="./node_modules/@webcomponents/webcomponentsjs/webcomponents-bundle.js"></script>
  <script type="module" src="my-app.js"></script>
</head>
<body>
  <my-app></my-app>
</body>

More typically, the app loads resources from an external file, as shown in the next example:

my-app.js

// Imports go here
class MyApp extends mixinBehaviors([AppLocalizeBehavior], PolymerElement) {
  static get template(){
    return html`
      <div>{{localize('hello', 'name', 'Batman')}}</div>
    `;
  }
  constructor() { 
    super();
    // set default language and localization strings
    this.language = 'en';
    this.resources = {
      'en': { 'hello': 'My name is {name}.' },
      'fr': { 'hello': 'Je m\'apelle {name}.' }
    };
  }
  attached: function() {
    this.loadResources(this.resolveUrl('locales.json'));
  },
}
customElements.define('my-app', MyApp);

The main app is also responsible for loading the Intl polyfill (not shown above).

Each element that needs to localize messages should also add AppLocalizeBehavior and use the localize method to translate strings, as shown above.