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

# Internationalization (i18n)

Angular's _internationalization_ (_i18n_) tools help make your app available in multiple languages.

Try this <live-example name="i18n" title="i18n Example in Spanish">live example</live-example>
of a JIT-compiled app, translated into Spanish.

{@a angular-i18n}

## 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.

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

Practitioners of _internationalization_ refer to a translatable text as a "_message_".
This page uses the words "_text_" and "_message_" interchangeably and in the combination, "_text message_".

</div>

The _i18n_ template translation process has four phases:

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

1. An angular _i18n_ tool extracts the marked messages into an industry standard translation source file.

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

1. 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.

{@a i18n-attribute}

## i18n pipes

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

By default Angular only contains locale data for the language `en-US`, if you set the value of
`LOCALE_ID` to another locale, you will have to import new locale data for this language:

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

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

Note that the files in `@angular/common/locales` contain most of the locale data that you will
need, but some advanced formatting options might only be available in the extra dataset that you can
import from `@angular/common/locales/extra`:

<code-example path="i18n/src/app/app.locale_data_extra.ts" region="import-locale-extra" title="src/app/app.locale_data_extra.ts" linenums="false">
</code-example>

</div>

## 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.


<div class="alert is-helpful">

`i18n` is not an Angular _directive_.
It's a custom _attribute_, recognized by Angular tools and compilers.
After translation, the compiler removes it.

</div>

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

<code-example path="i18n/src/app/app.component.1.html" region="greeting" title="src/app/app.component.html" linenums="false">
</code-example>

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

<code-example path="i18n/src/app/app.component.1.html" region="i18n-attribute" title="src/app/app.component.html" linenums="false">
</code-example>

{@a help-translator}

### Help the translator with a _description_ and _meaning_

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

<code-example path="i18n/src/app/app.component.1.html" region="i18n-attribute-desc" title="src/app/app.component.html" linenums="false">
</code-example>

In order to deliver a correct translation, the translator may need to
know the _meaning_ or _intent_ of the text within _this particular_ application context.

You add context by beginning the string with the _meaning_ and
separating it from the _description_ with the `|` character (`<meaning>|<description>`):

<code-example path="i18n/src/app/app.component.1.html" region="i18n-attribute-meaning" title="src/app/app.component.html" linenums="false">
</code-example>

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 facilitate contextually-specific translations.


{@a custom-id}

### Set a custom _id_ to improve search 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/src/locale/messages.es.xlf.html" region="generated-id" linenums="false">
</code-example>

This _id_ is obscure and difficult for humans to read or remember.

Worse, when you change the translatable text, perhaps to fix a typo,
the extractor tool generates a new _id_ for that translation.
You will lose the translation unless you update it with the new _id_.
That [complicates maintenance](#maintenance).

Consider specifying your own, meaningful _id_ in the `i18n` attribute, **prefixed with `@@`**.

<code-example path='i18n/src/app/app.component.1.html' region='i18n-attribute-solo-id' title='app/app.component.html' linenums="false">
</code-example>

Now the extractor tool and compiler will generate a translation unit with _your custom id_ and never change it.

<code-example path="i18n/src/locale/messages.es.xlf.html" region="custom-id" linenums="false">
</code-example>

Here is the `i18n` attribute with a _definition_, followed by the custom `id`:

<code-example path='i18n/src/app/app.component.1.html' region='i18n-attribute-id' title='app/app.component.html' linenums="false">
</code-example>

Here is a _meaning_ and a _description_ and the _id_ at the end:

<code-example path='i18n/src/app/app.component.1.html' region='i18n-attribute-meaning-and-id' title='app/app.component.html' linenums="false">
</code-example>

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

  Be sure to define _unique_ custom ids. If you use the same id for 2 _different_ blocks of text, only the first one will be extracted,
  and its translation used in both blocks of text.

  For example:

  ```html
    <p i18n="@@myId">Hello</p>
    <p i18n="@@myId">Good bye</p>
  ```

  with the translation:

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

  Both `<p>` elements will contain the text `Hola`.

</div>

{@a no-element}

### Translate text without creating an element

If there is a stretch of text that you'd like to translate, you could wrap it in a `<span>` tag.
But 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>` will be 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've added an image to your template. You care about accessibility too so you add a `title` attribute:

<code-example path="i18n/src/app/app.component.1.html" region="i18n-title" title="src/app/app.component.html" linenums="false">
</code-example>

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:

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

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

{@a cardinality}

## 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:

<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 (`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 (or any other number)
* zero
* one
* two
* few
* many
* 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}`.

<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>
that derives from the
<a href="http://cldr.unicode.org/" title="CLDR">Common Locale Data Repository (CLDR)</a>,
which specifies the
<a href="http://cldr.unicode.org/index/cldr-spec/plural-rules" title="Pluralization Rules">pluralization rules</a>.

</div>

{@a select}

## 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
<a href="http://userguide.icu-project.org/formatparse/messages" title="ICU Message Format">ICU message syntax</a>.
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:

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

## Nesting pluralization and selection expressions

You can also nest different ICU expressions together. For 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 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:

<code-example language="sh" class="code-shell">
  npm install @angular/compiler-cli @angular/platform-server --save
</code-example>

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

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

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

Windows users may have to quote the command like this: `"./node_modules/.bin/ng-xi18n"`

</div>

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>.

{@a other-formats}

### Other translation formats

Angular i18n tooling supports XLIFF 1.2 and XLIFF 2 as well as the
<a href="http://cldr.unicode.org/development/development-process/design-proposals/xmb" >XML Message Bundle (XMB)</a>.

You can specify your choice of format _explicitly_ with the `--i18nFormat` flag as illustrated in these example commands

<code-example language="sh" class="code-shell">
./node_modules/.bin/ng-xi18n  --i18nFormat=xlf  --outFile=messages.xlf
./node_modules/.bin/ng-xi18n  --i18nFormat=xlf2 --outFile=messages.xliff2.xlf
./node_modules/.bin/ng-xi18n  --i18nFormat=xmb  --outFile=messages.xmb
</code-example>

The sample in _this_ guide sticks with the default _XLIFF 1.2_ format.

{@a ng-xi18n-options}

### 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:

<code-example language="sh" class="code-shell">
  ./node_modules/.bin/ng-xi18n -p path/to/tsconfig.json
  ./node_modules/.bin/ng-xi18n  --i18nFormat=xmb -p path/to/tsconfig.json
</code-example>

{@a npm-i18n-script}

### 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:

<code-example format='.' language='sh'>
  "scripts": {
    "i18n": "ng-xi18n",
    ...
  }
</code-example>

Now you can issue command variations such as these:

<code-example language="sh" class="code-shell">
  npm run i18n
  npm run i18n -- -p path/to/tsconfig.json
  npm run i18n -- --i18nFormat=xmb -p path/to/tsconfig.json
</code-example>

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

{@a translate}

## 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 guide sample creates a Spanish translation file.

{@a localization-folder}

### 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,
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 suggestion. It has a `locale` folder under `src/`.
Assets within the folder carry a filename extension that matches a language-culture code from a
<a href="https://msdn.microsoft.com/en-us/library/ee825488(v=cs.20).aspx">well-known codeset</a>.

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

{@a translate-text-nodes}

### 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
<a href="https://en.wikipedia.org/wiki/XLIFF#Editors">many XLIFF file editors</a>.

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:

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

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

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

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.

</div>

Using the _source_, _description_, and _meaning_ elements to guide your translation,
replace the `<target/>` tag with the Spanish greeting:

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

Translate the other text nodes the same way:

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

<div class="alert is-important">

**The tool generated the `id`s for _these_ translation units. Don't touch them.**
Each `id` depends upon the content of the message and its assigned meaning.
Change either factor and the `id` changes as well.
See the **[translation file maintenance discussion](#maintenance)**.

This is why you should **[specify custom ids](#custom-id "Set a custom id")** and avoid tool generated ids.

</div>

{@a translate-plural-select}

## 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.
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.

{@a translate-plural}

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

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

{@a translate-select}

### Translate _select_
The `select` behaves a little differently. Here again is the ICU format message 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.

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.

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

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

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

Here they are together, after translation:

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

### Translate a nested expression

A nested expression is not different from the previous ones. As in the previous example, we have _two_ translation units.

The first one contains the text outside the nested expression:

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

The second unit contains the complete nested expression:

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

And both together:

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



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

<a id='app-pre-translation'></a>

### The app before translation

When the previous steps finish, the sample app _and_ its translation file are 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/src/main.1.ts">
  </code-pane>
  <code-pane title="src/locale/messages.es.xlf" path="i18n/src/locale/messages.es.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 application with the completed translation file.
The process is the same whether the file is in `.xlf` format or
in another format that Angular understands, such as `.xtb`.

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

  * The translation file.
  * The translation file format.
  * The   <a href="https://en.wikipedia.org/wiki/XLIFF">_Locale ID_</a>
  (`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](guide/i18n#jit), you provide the information at bootstrap time.
  * With [AOT](guide/i18n#aot), you pass the information as `ngc` options.

{@a jit}

### 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:

<code-example path="i18n/src/index.html" region="i18n" title="index.html (launch script)" linenums="false">
</code-example>

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

{@a text-plugin}

### SystemJS text plugin

<div class="alert is-important">

  This plugin only applies to an application using SystemJS. If you are using the Angular CLI, please refer to their
  [docs](https://github.com/angular/angular-cli/wiki/xi18n).

</div>

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:

<code-example path="i18n/src/systemjs-text-plugin.js" title="src/systemjs-text-plugin.js" linenums="false">
</code-example>

{@a create-translation-providers}

### 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`, `xlf2`, 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:

<code-example path="i18n/src/app/i18n-providers.ts" region="without-missing-translation" title="src/app/i18n-providers.ts">
</code-example>

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

1. 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.

1. It creates a transaction filename from the locale according to the name and location convention
[described earlier](guide/i18n#localization-folder).

1. 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](guide/i18n#text-plugin).

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

1. Finally, `getTranslationProviders()` returns the entire effort as a promise.

<div class="alert is-important">

  The `LOCALE_ID` has to be a valid locale id as explained in [here](http://userguide.icu-project.org/locale).

</div>

{@a bootstrap-the-app}

### Bootstrap 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:

<code-example path="i18n/src/main.ts" title="src/main.ts" linenums="false">
</code-example>

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.

{@a aot}

### _Internationalization_ 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, in this case `index.html`,
you determine which language the user needs
and serve the appropriate application package.

This guide 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](guide/i18n#app-pre-translation)
and refer to the [AOT guide](guide/aot-compiler) 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:

<code-example language="sh" class="code-shell">
  ./node_modules/.bin/ngc --i18nFile=./locale/messages.es.xlf --locale=es --i18nFormat=xlf
</code-example>

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

Windows users may have to quote the command:

<code-example language="sh" class="code-shell">
  "./node_modules/.bin/ngc" --i18nFile=./locale/messages.es.xlf --locale=es --i18nFormat=xlf
</code-example>

</div>

### Report missing translations

If you forgot to provide a translation, the build will succeed with a warning that you might easily overlook.
You can configure the Angular compiler for different "missing translation" behaviors:

* Error
* Warning (default)
* Ignore

To change the behavior in JIT, you can use the following configuration:

<code-example language="typescript">
  { provide: CompilerConfig, useValue: new CompilerConfig({ missingTranslation: MissingTranslationStrategy.Error }) }
</code-example>

A good place to use it is the translation providers:

<code-example path="i18n/src/app/i18n-providers.ts" region="missing-translation" title="src/app/i18n-providers.ts"></code-example>

To change the behavior in AOT, add the `--missingTranslation` flag to the compilation command:

<code-example language="sh" class="code-shell">
  ./node_modules/.bin/ngc --i18nFile=./locale/messages.es.xlf --locale=es --i18nFormat=xlf --missingTranslation=error
</code-example>

{@a maintenance}

## 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 the `id` _can be a serious problem!_

If the `id` is generated by the tool, _most_ changes to _existing_ markup
cause the tool to generate a _new_ `id` for the affected translation unit.

After an `id` changes, the translation files are no longer in sync.
Because of that, you get some warning messages during re-compilation.
The warning messages identify that some translations are missing, but they don't tell you which
old `ids` are no longer valid.

If you use a [custom id](#custom-id "Set a custom id"),
the tooling preserves the custom `id` as you make changes to the corresponding translation unit. **Use custom _ids_ unless you have a very good reason to do otherwise.**

Whether you use generated or custom `ids`, **always 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
will help you find and update `ids` and other changes across your translation files.