angular-docs-cn/aio/content/guide/i18n.md

# Internationalization (i18n)

Application internationalization is a many-faceted area of development, focused on making
applications available and user-friendly to a worldwide audience. This page describes Angular's
internationalization (i18n) tools, which can help you make your app available in multiple languages.

See the <live-example downloadOnly name="i18n">i18n Example</live-example> for a simple example of
an AOT-compiled app, translated into French.

{@a angular-i18n}
## Angular and i18n

Angular simplifies the following aspects of internationalization:
* Displaying dates, number, percentages, and currencies in a local format.
* Translating text in component templates.
* Handling plural forms of words.
* Handling alternative text.

This document focuses on [**Angular CLI**](https://cli.angular.io/) projects, in which the Angular
CLI generates most of the boilerplate necessary to write your app in multiple languages.

{@a setting-up-locale}
## Setting up the locale of your app

A locale is an identifier (id) that refers to a set of user preferences that tend to be shared
within a region of the world, such as country. This document refers to a locale identifier as a
"locale" or "locale id".

A Unicode locale identifier is composed of a Unicode language identifier and (optionally) the
character `-` followed by a locale extension. (For historical reasons the character `_` is supported
as an alternative to `-`.)  For example, in the locale id `fr-CA` the `fr` refers to the French
language identifier, and the `CA` refers to the locale extension Canada.

<div class="alert is-critical">

Angular follows the Unicode LDML convention that uses stable identifiers (Unicode locale identifiers)
based on the norm [BCP47](http://www.rfc-editor.org/rfc/bcp/bcp47.txt). It is very important that
you follow this convention when you define your locale, because the Angular i18n tools use this
locale id to find the correct corresponding locale data.

</div>

By default, Angular uses the locale `en-US`, which is English as spoken in the United States of America.

To set your app's locale to another value, use the CLI parameter `--locale` with the value
of the locale id that you want to use:

<code-example language="sh" class="code-shell">
  ng serve --aot --locale fr
</code-example>

If you use JIT, you also need to define the `LOCALE_ID` provider in your main module:

<code-example path="i18n/doc-files/app.module.ts" title="src/app/app.module.ts" linenums="false">
</code-example>


For more information about Unicode locale identifiers, see the 
[CLDR core spec](http://cldr.unicode.org/core-spec#Unicode_Language_and_Locale_Identifiers).

For a complete list of locales supported by Angular, see 
[the Angular repository](https://github.com/angular/angular/tree/master/packages/common/locales).

The locale identifiers used by CLDR and Angular are based on [BCP47](http://www.rfc-editor.org/rfc/bcp/bcp47.txt).
These specifications change over time; the following table maps previous identifiers to current ones at
time of writing:

| Locale name                   | Old locale id     | New locale id |
|-------------------------------|-------------------|---------------|
| Indonesian                    | in                | id            |
| Hebrew                        | iw                | he            |
| Romanian Moldova              | mo                | ro-MD         |
| Norwegian Bokmål              | no, no-NO         | nb            |
| Serbian Latin                 | sh                | sr-Latn       |
| Filipino                      | tl                | fil           |
| Portuguese Brazil             | pt-BR             | pt            |
| Chinese Simplified            | zh-cn, zh-Hans-CN | zh-Hans       |
| Chinese Traditional           | zh-tw, zh-Hant-TW | zh-Hant       |
| Chinese Traditional Hong Kong | zh-hk             | zh-Hant-HK    |


## i18n pipes

Angular pipes can help you with internationalization: the `DatePipe`, `CurrencyPipe`, `DecimalPipe`
and `PercentPipe` use locale data to format data based on the `LOCALE_ID`.

By default, Angular only contains locale data for `en-US`. If you set the value of
`LOCALE_ID` to another locale, you must import locale data for that new locale.
The CLI imports the locale data for you when you use the parameter `--locale` with `ng serve` and
`ng build`.

If you want to import locale data for other languages, you can do it manually:

<code-example path="i18n/doc-files/app.locale_data.ts" region="import-locale" title="src/app/app.module.ts" linenums="false">
</code-example>

The first parameter is an object containing the locale data imported from `@angular/common/locales`.
By default, the imported locale data is registered with the locale id that is defined in the Angular
locale data itself.
If you want to register the imported locale data with another locale id, use the second parameter to
specify a custom locale id. For example, Angular's locale data defines the locale id for French as
"fr". You can use the second parameter to associate the imported French locale data with the custom
locale id "fr-FR" instead of "fr".

The files in `@angular/common/locales` contain most of the locale data that you 
need, but some advanced formatting options might only be available in the extra dataset that you can
import from `@angular/common/locales/extra`. An error message informs you when this is the case.
  
  <code-example path="i18n/doc-files/app.locale_data_extra.ts" region="import-locale-extra" title="src/app/app.module.ts" linenums="false">
  </code-example>

<div class="l-sub-section">

  All locale data used by Angular are extracted from the Unicode Consortium's
  <a href="http://cldr.unicode.org/" title="CLDR">Common Locale Data Repository (CLDR)</a>.

</div> 


## Template translations

<div class="l-sub-section">

  This document refers to a unit of translatable text as "text," a "message", or a 
  "text message." 

</div>

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 text into an industry standard translation source file.

3. A translator edits that file, translating the extracted text 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 app
in the target language.

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

{@a i18n-attribute}
### Mark text with the i18n attribute

The Angular `i18n` attribute marks translatable content. Place it on every element tag whose fixed
text is to be translated.

In the example below, an `<h1>` tag displays a simple English language greeting, "Hello i18n!"

<code-example path="i18n/doc-files/app.component.html" region="greeting" title="src/app/app.component.html" linenums="false">
</code-example>

To mark the greeting for translation, add the `i18n` attribute to the `<h1>` tag.

<code-example path="i18n/doc-files/app.component.html" region="i18n-attribute" title="src/app/app.component.html" linenums="false">
</code-example>


<div class="alert is-helpful">

  `i18n` is a custom attribute, recognized by Angular tools and compilers.
  After translation, the compiler removes it. It is not an Angular directive.

</div>


{@a help-translator}
### Help the translator with a description and meaning

To translate a text message accurately, the translator may need additional information or context. 

You can add a description of the text message as the value of the `i18n` attribute, as shown in the
example below:

<code-example path="i18n/doc-files/app.component.html" region="i18n-attribute-desc" title="src/app/app.component.html" linenums="false">
</code-example>

The translator may also need to know the meaning or intent of the text message within this particular 
app context.

You add context by beginning the `i18n` attribute value with the _meaning_ and
separating it from the _description_ with the `|` character: `<meaning>|<description>`

<code-example path="i18n/doc-files/app.component.html" region="i18n-attribute-meaning" title="src/app/app.component.html" linenums="false">
</code-example>

All occurrences of a text message that have the same meaning will have the same translation.
A text message that is associated with different meanings can have different translations.

The Angular extraction tool preserves both the meaning and the description in the translation
source file to facilitate contextually-specific translations, but only the combination of meaning
and text message are used to generate the specific id of a translation. If you have two
similar text messages with different meanings, they are extracted separately. If you have two similar
text messages with different descriptions (not different meanings), then they are extracted only once. 


{@a custom-id}
### Set a custom id for persistence and maintenance

The angular i18n extractor tool generates a file with a translation unit entry for each `i18n`
attribute in a template. By default, it assigns each translation unit a unique id such as this one:

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="generated-id" linenums="false">
</code-example>

When you change the translatable text, the extractor tool generates a new id for that translation unit.
You must then update the translation file with the new id.

Alternatively, you can specify a custom id in the `i18n` attribute by using the prefix `@@`.
The example below defines the custom id `introductionHeader`: 

<code-example path='i18n/doc-files/app.component.html' region='i18n-attribute-solo-id' title='app/app.component.html' linenums="false">
</code-example>

When you specify a custom id, the extractor tool and compiler generate a translation unit with that
custom id.

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="custom-id" linenums="false">
</code-example>

The custom id is persistent. The extractor tool does not change it when the translatable text changes.
Therefore, you do not need to update the translation. This approach makes maintenance easier. 

#### Use a custom id with a description

You can use a custom id in combination with a description by including both in the value of the
`i18n` attribute. In the example below, the `i18n` attribute value includes a description, followed
by the custom `id`:

<code-example path='i18n/doc-files/app.component.html' region='i18n-attribute-id' title='app/app.component.html' linenums="false">
</code-example>

You also can add a meaning, as shown in this example:

<code-example path='i18n/doc-files/app.component.html' region='i18n-attribute-meaning-and-id' title='app/app.component.html' linenums="false">
</code-example>

#### Define unique custom ids

Be sure to define custom ids that are unique. If you use the same id for two different text messages,
only the first one is extracted, and its translation is used in place of both original text messages.

In the example below the custom id `myId` is used for two different messages:

  ```html
    <h3 i18n="@@myId">Hello</h3>
    <!-- ... -->
    <p i18n="@@myId">Good bye</p>
  ```

Consider this translation to French:

  ```xml
    <trans-unit id="myId" datatype="html">
      <source>Hello</source>
      <target state="new">Bonjour</target>
    </trans-unit>
  ```

Because the custom id is the same, both of the elements in the resulting translation contain
the same text, `Bonjour`:

  ```html
    <h3>Bonjour</h3>
    <!-- ... -->
    <p>Bonjour</p>
  ```
 


{@a no-element}
### Translate text without creating an element

If there is a section of text that you would like to translate, you can wrap it in a `<span>` tag.
However, if you don't want to create a new DOM element merely to facilitate translation,
you can wrap the text in an `<ng-container>` element.
The `<ng-container>` is transformed into an html comment:

<code-example path="i18n/src/app/app.component.html" region="i18n-ng-container" title="src/app/app.component.html" linenums="false">
</code-example>

{@a translate-attributes}
## Add i18n translation attributes

You also can translate attributes.
For example, assume that your template has an image with a `title` attribute:

<code-example path="i18n/doc-files/app.component.html" region="i18n-title" title="src/app/app.component.html" linenums="false">
</code-example>

This `title` attribute needs to be translated.

To mark an attribute for translation, add an attribute in the form of `i18n-x`,
where `x` is the name of the attribute to translate. The following example shows how to mark the
`title` attribute for translation by adding the `i18n-title` attribute on the `img` tag:

<code-example path="i18n/src/app/app.component.html" region="i18n-title-translate" title="src/app/app.component.html" linenums="false">
</code-example>

This technique works for any attribute of any element.

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

{@a plural-ICU}
## Translate singular and plural

Different languages have different pluralization rules.

Suppose that you want to say that something was "updated x minutes ago".
In English, depending upon the number of minutes, you could display "just now", "one minute ago",
or "x minutes ago" (with x being the actual number).
Other languages might express the cardinality differently.

The example below shows how to use a `plural` ICU expression to display one of those three options
based on when the update occurred:

<code-example path="i18n/src/app/app.component.html" region="i18n-plural" title="src/app/app.component.html" linenums="false">
</code-example>

* The first parameter is the key. It is bound to the component property (`minutes`), which determines
the number of minutes.
* 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.


<div class="l-sub-section">

  This syntax conforms to the
  <a href="http://userguide.icu-project.org/formatparse/messages" title="ICU Message Format">ICU Message Format</a>
  as specified in the
  <a href="http://cldr.unicode.org/index/cldr-spec/plural-rules" title="Pluralization Rules">CLDR pluralization rules</a>.

</div>

Pluralization categories include (depending on the language):

* =0 (or any other number)
* zero
* one
* two
* few
* many
* other

After the pluralization category, put the default English text in braces (`{}`).

In the example above, the three options are specified according to that pluralization pattern. For
talking about about zero minutes, you use `=0 {just now}`. For one minute, you use `=1 {one minute}`.
Any unmatched cardinality uses `other {{{minutes}} minutes ago}`. You could choose to add patterns
for two, three, or any other number if the pluralization rules were different. For the example of
"minute", only these three patterns are necessary in English.

<div class="l-sub-section">

  You can use interpolations and html markup inside of your translations.

</div>

{@a select-ICU}
## Select among alternative text messages

If your template needs to display different text messages depending on the value of a variable, you
need to translate all of those alternative text messages.

You can handle this with a `select` ICU expression. It is similar to the `plural` ICU expressions
except that you choose among alternative translations based on a string value instead of a number,
and you define those string values.

The following format message in the component template binds to the component's `gender` property,
which outputs one of the following string values: "m", "f" or "o".
The message maps those values to the appropriate translations:

<code-example path="i18n/src/app/app.component.html" region="i18n-select" title="src/app/app.component.html" linenums="false">
</code-example>

{@a nesting-ICUS}
## Nesting plural and select ICU expressions

You can also nest different ICU expressions together, as shown in this example:

<code-example path="i18n/src/app/app.component.html" region="i18n-nested" title="src/app/app.component.html">
</code-example>

{@a ng-xi18n}
## Create a translation source file with _ng xi18n_

Use the `ng xi18n` command provided by the CLI to extract the text messages marked with `i18n` into
a translation source file.

Open a terminal window at the root of the app project and enter the `ng xi18n` command:

<code-example language="sh" class="code-shell">
  ng xi18n
</code-example>

By default, the tool generates a translation file named `messages.xlf` in the
<a href="https://en.wikipedia.org/wiki/XLIFF">XML Localization Interchange File Format
(XLIFF, version 1.2)</a>.

<div class="l-sub-section">

If you don't use the CLI, you have two options:
* You can use the `ng-xi18n` tool directly from the `@angular/compiler-cli` package.
For more information, see [i18n in the CLI documentation](https://github.com/angular/angular-cli/wiki/xi18n).
* You can use the CLI Webpack plugin `AngularCompilerPlugin` from the `@ngtools/webpack` package.
Set the parameters `i18nOutFile` and `i18nOutFormat` to trigger the extraction.
For more information, see the [Angular Ahead-of-Time Webpack Plugin documentation](https://github.com/angular/angular-cli/tree/master/packages/%40ngtools/webpack).

</div>

{@a other-formats}
### Other translation formats

Angular i18n tooling supports three translation formats:
* XLIFF 1.2 (default)
* XLIFF 2
* <a href="http://cldr.unicode.org/development/development-process/design-proposals/xmb" >XML Message
Bundle (XMB)</a>

You can specify the translation format explicitly with the `--i18nFormat` flag as illustrated in
these example commands:

<code-example language="sh" class="code-shell">
ng xi18n  --i18nFormat=xlf
ng xi18n  --i18nFormat=xlf2
ng xi18n  --i18nFormat=xmb
</code-example>

The sample in this guide uses the default XLIFF 1.2 format.

<div class="l-sub-section">

  XLIFF files have the extension .xlf. The XMB format generates .xmb source files but uses
  .xtb (XML Translation Bundle: XTB) translation files.

</div>

{@a ng-xi18n-options}
### Other options

You can specify the output path used by the CLI to extract your translation source file with
the parameter `--outputPath`:

<code-example language="sh" class="code-shell">

  ng xi18n --outputPath src/locale

</code-example>

You can change the name of the translation source file that is generated by the extraction tool with
the parameter `--outFile`:

<code-example language="sh" class="code-shell">

  ng xi18n --outFile source.xlf

</code-example>

You can specify the base locale of your app with the parameter `--locale`:

<code-example language="sh" class="code-shell">

  ng xi18n --locale fr

</code-example>

The extraction tool uses the locale to add the app locale information into your translation source
file. This information is not used by Angular, but external translation tools may need it.


{@a translate}
## Translate text messages

The `ng xi18n` command generates a translation source file named `messages.xlf` in the project `src`
folder. 

The next step is to translate this source file into the specific language
translation files. The example in this guide creates a French translation file.

{@a localization-folder}
### Create a localization folder

Most apps are translated into more than one other language. For this reason, it is standard practice
for the project structure to reflect the entire internationalization effort.

One approach is to dedicate a folder to localization and store related assets, such as
internationalization files, there.

<div class="l-sub-section">

  Localization and internationalization are
  <a href="https://en.wikipedia.org/wiki/Internationalization_and_localization">different but
  closely related terms</a>.

</div>

This guide follows that approach. It has a `locale` folder under `src/`.
Assets within that folder have a filename extension that matches their associated locale.

### Create the translation files

For each translation source file, there must be at least one language translation file for the
resulting translation.

For this example:

1. Make a copy of the `messages.xlf` file.
2. Put the copy in the `locale` folder.
3. Rename the copy to `messages.fr.xlf` for the French language translation.

If you were translating to other languages, you would repeat these steps for each target language.

{@a translate-text-nodes}
### Translate text nodes

In a large translation project, you would send the `messages.fr.xlf` file to a French translator who
would enter the translations using an XLIFF file editor.

This sample file is easy to translate without a special editor or knowledge of French.

1. Open `messages.fr.xlf` and find the first `<trans-unit>` section:

> <code-example path="i18n/doc-files/messages.fr.xlf.html" region="translated-hello-before" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

> This XML element represents the translation of the `<h1>` greeting tag that you marked with the
  `i18n` attribute earlier in this guide. 

> Note that the translation unit `id=introductionHeader` is derived from the
  [custom `id`](#custom-id "Set a custom id") that you set earlier, but
  without the `@@` prefix required in the source HTML.


2. Duplicate the `<source/>` tag, rename it `target`, and then replace its content with the French
  greeting. If you were working with a more complex translation, you could use the the information
  and context provided by the source, description, and meaning elements to guide your selection of
  the appropriate French translation.

> <code-example path="i18n/doc-files/messages.fr.xlf.html" region="translated-hello" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;, after translation)" linenums="false">
</code-example>

3. Translate the other text nodes the same way:

> <code-example path="i18n/doc-files/messages.fr.xlf.html" region="translated-other-nodes" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

<div class="alert is-important">

  **The Angular i18n tools generated the ids for these translation units. Don't change them.**
  Each `id` depends upon the content of the template text and its assigned meaning.
  If you change either the text or the meaning, then the `id` changes.
  For more information, see the **[translation file maintenance discussion](#custom-id)**.

</div>

{@a translate-plural-select}
## Translate plural and select expressions

_Plural_ and _select_ ICU expressions are extracted separately, so they require special attention
when preparing for translation.

Look for these expressions 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.

{@a translate-plural}
### Translate _plural_

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

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="translated-plural" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

You can add or remove plural cases, with each language having its own cardinality.  (See 
[CLDR plural rules](http://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html).)

{@a translate-select}
### Translate _select_

Below is the content of our example `select` ICU expression in the component template:

<code-example path="i18n/src/app/app.component.html" region="i18n-select" title="src/app/app.component.html" linenums="false">
</code-example>

The extraction tool broke that into two translation units because ICU expressions are extracted
separately.

The first unit contains the text that was outside of the `select`.
In place of the `select` is a placeholder, `<x id="ICU">`, that represents the `select` message.
Translate the text and move around the placeholder if necessary, but don't remove it. If you remove
the placeholder, the ICU expression will not be present in your translated app.

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="translate-select-1" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

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

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="translate-select-2" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

Here they are together, after translation:

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="translated-select" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

{@a translate-nested}
### Translate a nested expression

A nested expression is similar to the previous examples. As in the previous example, there are 
two translation units. The first one contains the text outside of the nested expression:

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="translate-nested-1" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

The second unit contains the complete nested expression:

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="translate-nested-2" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

And both together:

<code-example path="i18n/doc-files/messages.fr.xlf.html" region="translate-nested" title="src/locale/messages.fr.xlf (&lt;trans-unit&gt;)" linenums="false">
</code-example>

The entire template translation is complete. The next section describes how to load that translation
into the app.

{@a app-pre-translation}
### The app and its translation file

The sample app and its translation file are now as follows:

<code-tabs>
  <code-pane title="src/app/app.component.html" path="i18n/src/app/app.component.html">
  </code-pane>
  <code-pane title="src/app/app.component.ts" path="i18n/src/app/app.component.ts">
  </code-pane>
  <code-pane title="src/app/app.module.ts" path="i18n/src/app/app.module.ts">
  </code-pane>
  <code-pane title="src/main.ts" path="i18n/doc-files/main.1.ts">
  </code-pane>
  <code-pane title="src/locale/messages.fr.xlf" path="i18n/doc-files/messages.fr.xlf.html">
  </code-pane>
</code-tabs>

{@a merge}
## Merge the completed translation file into the app

To merge the translated text into component templates, compile the app with the completed
translation file. Provide the Angular compiler with three translation-specific pieces of information:

  * The translation file.
  * The translation file format.
  * The locale (`fr` or `en-US` for instance).

The compilation process is the same whether the translation file is in `.xlf` format or in another
format that Angular understands, such as `.xtb`.

How you provide this information depends upon whether you compile with
the JIT compiler or the AOT compiler.

  * With [AOT](guide/i18n#merge-aot), you pass the information as a CLI parameter.
  * With [JIT](guide/i18n#merge-jit), you provide the information at bootstrap time.


{@a merge-aot}
### Merge with the AOT compiler

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 must pre-build a separate application
package for each language and serve the appropriate package based on either server-side language
detection or url parameters.

You also need to instruct the AOT compiler to use your translation file. To do so, you use three
options with the `ng serve` or `ng build` commands:

* `--i18nFile`: the path to the translation file.
* `--i18nFormat`: the format of the translation file.
* `--locale`: the locale id.

The example below shows how to serve the French language file created in previous sections of this
guide:

<code-example language="sh" class="code-shell">
  ng serve --aot --i18nFile=src/locale/messages.fr.xlf --i18nFormat=xlf --locale=fr
</code-example>

{@a merge-jit}
### Merge with the JIT compiler

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

1. Importing the appropriate language translation file as a string constant.
2. Creating corresponding translation providers for the JIT compiler.
3. Bootstrapping the app with those providers.

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

* `TRANSLATIONS` is a string containing the content of the translation file.
* `TRANSLATIONS_FORMAT` is the format of the file: `xlf`, `xlf2`, or `xtb`.
* `LOCALE_ID` is the locale of the target language.

The Angular `bootstrapModule` method has a second `compilerOptions` parameter that can influence the
behavior of the compiler. You can use it to provide the translation providers:

<code-example path="i18n/doc-files/main.2.ts" title="src/main.ts">
</code-example>

Then provide the `LOCALE_ID` in the main module:

<code-example path="i18n/doc-files/app.module.ts" title="src/app/app.module.ts" linenums="false">
</code-example>


{@a missing-translation}
### Report missing translations
By default, when a translation is missing, the build succeeds but generates a warning such as
`Missing translation for message "foo"`. You can configure the level of warning that is generated by
the Angular compiler:

* Error: throw an error. If you are using AOT compilation, the build will fail. If you are using JIT
compilation, the app will fail to load.
* Warning (default): show a 'Missing translation' warning in the console or shell.
* Ignore: do nothing.

If you use the AOT compiler, specify the warning level by using the CLI parameter
`--missingTranslation`. The example below shows how to set the warning level to error:

<code-example language="sh" class="code-shell">
  ng serve --aot --missingTranslation=error
</code-example>

If you use the JIT compiler, specify the warning level in the compiler config at bootstrap by adding
the 'MissingTranslationStrategy' property. The example below shows how to set the warning level to
error:

<code-example path="i18n/doc-files/main.3.ts" title="src/main.ts">
</code-example>