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# Introduction

All CKEditor 5 WYSIWYG editor features support message localization. It means that the user interface of any feature can be translated into various languages and regions depending on the user’s preferences.

CKEditor 5 translation system is open to third-party plugins. Any custom features that you introduce can be localized. The system also provides a way to add missing or overwrite existing translations and supports translating plural forms.

Make sure to use up-to-date CKEditor 5 development tool packages. Versions of the tools older than v19.0.1 do not provide support for features described in this guide.

# Open translation API

The CKEditor 5 localization system focuses on the following points:

  • Supporting the localization of third-party plugins.
  • Making it possible to pass your own translations to fix missing or invalid localizations.
  • Generating deterministic builds.
  • Exposing easy-to-use APIs for providing translations and writing localizable content.
  • Supporting plural forms in each step of the localization system for better translations.

# Glossary of terms

Before we start, let us explain the meaning of terms that are crucial for the translation process:

  • A message – A string or an object that should be translated.
    The string version works as a shortcut for the { id: message, string: message } object form.
  • A message ID – A property used to distinguish messages.
    It is useful for short messages where a collision might occur, like %0 images.
  • A message string – The default (English) form of the message.
    When the message supports plural versions, this is the default singular version.
  • A message plural – An optional plural (English) version of the message.
    The presence of this property indicates that the message should support both singular and plural forms.
  • A PO file (.po) – A file containing translations for the language in the PO format.
    All localizable packages of CKEditor 5 contain such files in the lang/translations/ directory.
  • A POT file (.pot) – A file containing source messages (English sentences) that will be translated.
  • A translation asset – A JavaScript file or a part of the file with generated translations for one language.

# Writing a localizable UI

All messages that need localization should be passed to the special CKEditor 5’s t() function. This function can be retrieved either from the editor’s Locale instance: const { t } = editor.locale; or from any view method: const t = this.t;.

As the first argument, the t() function accepts either a string literal or an object literal containing the id, string and plural (optional) properties. The string literal will serve as both the message ID and the message string.

As the second argument, the translation function accepts a value or an array of values. These values will be used to fill the placeholders in more advanced translation scenarios. If the plural property is specified, the first value will be used as the quantity determining the plural form.

Due to the fact that a static code analyzer is used in the translation process, you must use a function named exactly t(). It should not be called on a Locale instance and it cannot have a different name. The code analyzer looks for localizable messages only in t() function calls.

For the same reason, the first argument can only be a string literal or an object literal. Variables cannot be passed.

When using the t() function, you can create your own localizable messages or reuse messages created in CKEditor 5 packages that your project depends on. In case of reusing messages, you will not need to worry about translating them as all work will be done by the CKEditor 5 team and Transifex translators. Obviously, your help in translating will still be appreciated!

For simple localizable messages, use the string form for simplicity:

const emojiName = 'cat';

// Assuming that the English language was picked:
t( 'insert emoji' ); // "insert emoji"
t( 'insert %0 emoji', emojiName ); // "insert cat emoji"
t( 'insert %0 emoji', [ emojiName ] ); // "insert cat emoji"

For more advanced scenarios, use plain object forms:

const quantity = 3;

// Assuming that the English language was picked:
t( { string: '%0 emoji', id: 'ACTION_EMOJI' }, 'insert' ); // "insert emoji"
t( { string: '%0 emoji', plural: '%0 emojis', id: 'N_EMOJIS' }, quantity ); // "3 emojis"
t( { string: '%1 %0 emoji', plural: '%1 %0 emojis', id: 'ACTION_N_EMOJIS' }, [ quantity, 'Insert' ] ); // "Insert 3 emojis"

# Example: Localizing the plugin UI

This example shows how to create a localizable user interface of a plugin. Here is how you can create a button that will insert a smiling face emoji. The button will have a localizable tooltip.

// Custom plugin configuration, including necessary imports.
// The code below should be put into a custom plugin class extending the Plugin class.
// ...

editor.ui.componentFactory.add( 'smilingFaceEmoji', locale => {
    const buttonView = new ButtonView( locale );

    // The translation function.
    const { t } = editor.locale;

    // The localized label.
    const label = t( 'Insert smiling face emoji' );

    buttonView.set( {
        icon: emojiIcon,
        tooltip: true
    } );

    buttonView.on( 'execute', () => {
        editor.execute( 'insertSmilingFaceEmoji' );
    } );
} );

// The rest of the custom plugin configuration.
// ...

See how to create a complete plugin to have a better understanding of creating CKEditor 5 plugins.

# Example: Localizing pending actions

Pending actions are used to inform the user that an action is in progress and they will lose data if they exit the editor at the given moment. Here is how you can localize them:

class FileRepository {
    // More methods.
    // ...

    updatePendingAction() {
        const pendingActions = this.editor.plugins.get( PendingActions );

        const t = this.editor.t;
        const getMessage = value => t( 'Upload in progress (%0%).', value ); // Upload in progress (12%).

        this._pendingAction = pendingActions.add( getMessage( this.uploadedPercent ) );
        this._pendingAction.bind( 'message' ).to( this, 'uploadedPercent', getMessage );

# Adding translations and localizing the editor UI

First of all, if you found a missing or incorrect translation in any of CKEditor 5 features, see how you can contribute to the project. CKEditor 5 is an Open Source project used by people from all around the world, so your help will be appreciated by others.

Adding translations to the editor can be done in three ways to satisfy various needs.

# Using the add() function

The first option for adding translations is via the translation-service’s add() helper. This utility adds translations to the global window.CKEDITOR_TRANSLATIONS object by extending it. Since it needs to be imported, it works only before building the editor.

Starting with the CKEditor 5 v19.0.0 release, the add() method now accepts an optional getPluralForm() function as the third argument. This function is only needed for defining the plural form if no language file was loaded for a particular language. It also accepts an array of translations for a message if the message should support singular and plural forms.

add( 'pl', {
    'Add space': [ 'Dodaj spację', 'Dodaj %0 spacje', 'Dodaj %0 spacji' ]
} );

// Assuming that the Polish language was picked:
t( { string: 'Add space', plural: 'Add %0 spaces' }, 1 ) // "Dodaj spację"
t( { string: 'Add space', plural: 'Add %0 spaces' }, 2 ) // "Dodaj 2 spacje"
t( { string: 'Add space', plural: 'Add %0 spaces' }, 5 ) // "Dodaj 5 spacji"

# Using the window.CKEDITOR_TRANSLATIONS object

The second option is adding translations via the global window.CKEDITOR_TRANSLATIONS object.

For each language that should be supported, the dictionary property of this object should be extended and the getPluralForm() function should be provided if missing.

The dictionary property is a message ID ⇒ translations map, where the translations can be either a string or an array of translations with plural forms for the given language if the message should support plural forms as well.

The getPluralForm() property should be a function that returns the plural form index for a given quantity. Note that when using CKEditor 5 translations, this property will be defined by CKEditor 5 translation assets.

Check an example below that demonstrates a part of the window.CKEDITOR_TRANSLATIONS object with Polish translations for the Cancel and Add space message IDs:

    // Each key should be a valid language code.
    pl: {
        // A map of translations for the 'pl' language.
        dictionary: {
            'Cancel': 'Anuluj',
            'Add space': [ 'Dodaj spację', 'Dodaj %0 spacje', 'Dodaj %0 spacji' ]

        // A function that returns the plural form index for the given language.
        // Note that you only need to pass this function when you add translations for a new language.
        getPluralForm: n => n == 1 ? 0 : n % 10 >= 2 && n % 10 <= 4 && ( n % 100 < 10 || n % 100 >= 20 ) ? 1 : 2
    // Other languages.
    // ...

It is important to extend the existing properties in the window.CKEDITOR_TRANSLATIONS object to not lose other translations. This can be achieved easily using Object.assign() and the || operator.

// Make sure that the global object is defined. If not, define it.

// Make sure that the dictionary for Polish translations exist.
window.CKEDITOR_TRANSLATIONS[ 'pl' ] = window.CKEDITOR_TRANSLATIONS[ 'pl' ] || {};
window.CKEDITOR_TRANSLATIONS[ 'pl' ].dictionary =  window.CKEDITOR_TRANSLATIONS[ 'pl' ].dictionary || {};

// Extend the dictionary for Polish translations with your translations:
Object.assign( window.CKEDITOR_TRANSLATIONS[ 'pl' ].dictionary, {
    'Save': 'Zapisz'
} );

If you add a new language, remember to set the getPluralForm() function which should return a number (or a Boolean for languages with simple plural rules like English) that indicates which form should be used for the given value.

# Creating .po files

The third option of adding plugins should fit mostly owners of plugins that contain many localizable messages. Using this option you need to create a .po file per each language code in the lang/translations/ directory containing translations that follow the PO file format.

# lang/translations/es.po

msgid ""
msgstr ""
"Language: es\n"
"Plural-Forms: nplurals=2; plural=(n != 1);\n"

msgid "Align left"
msgstr "Alinear a la izquierda"

Note that by default, the CKEditor 5 translations plugin is configured to parse only the CKEditor 5 source code when looking for localizable messages and generating translation assets.

If you develop your own plugin outside the CKEditor 5 ecosystem and localize it by creating PO files, you should override both the sourceFilesPattern and the packageNamePattern options to allow the CKEditor 5 webpack plugin to analyze the code and find messages with corresponding translations. You should also mention these webpack plugin changes in your package README to make other users build the localized CKEditor 5 editor with your plugin correctly. This obstacle may be simplified in the future when the localization feature gets more popular.

To build and configure a localized editor, follow the steps from the Setting the UI language guide.

# Re-using translations from other packages

If you want to re-use a message that already exists in another package, you should use the method t() on a Locale instance with a changed name instead of using t() as a function. This is because using t() as a method on the Locale is not processed by a static code analyzer. Therefore, it allows to use messages already translated in other packages.

We use this approach already in the collaboration features and the slash commands feature. You can find an example from the list of default commands that we used in the slash commands feature below. Please note the difference between using t() and translateVariableKey(). translateVariableKey( 'Block quote' ) will re-use a translation from another package whilst t( 'Create a block quote' ) will be processed by a static code analyzer. As a result we make sure that the translation for the title is taken from the block quote feature where the message “Block quote” is already translated. But for the description we create a new translation.

public getDefaultCommands() {
    const t = this.editor.t;
    const translateVariableKey = this.editor.locale.t;

    return [
            id: 'blockQuote',
            commandName: 'blockQuote',
            icon: icons.quote,
            title: translateVariableKey( 'Block quote' ),
            description: t( 'Create a block quote' )
        // More command definitions
        // ...

# Known limitations

  • Currently it is impossible to change the chosen editor’s language at runtime without destroying the editor.
  • Currently it is impossible to add more than one language to the bundle using the the CKEditor 5 webpack plugin. In case where multiple translation assets should be added to the application, they should be added using the <script> tags or imports to the generated translation assets.