angular-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.

可以把这个翻译为法语版的 AOT 应用<live-example name="i18n">i18n 例子</live-example>作为一个简单的例子。


{@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.

**国际化**工作者通常将一个可翻译的文本叫作“信息”。
本章使用了“文本”和“信息”，它们可以互换，也可以组合“文本信息”。

</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.


添加`i18n`属性到该标签上，把它标记为需要翻译的文本。

<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.

为了给出正确的翻译，翻译者需要知道你这段文本在特定情境下的 *真实意图*。

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

在描述的前面，我们为指定的字符串添加一些上下文含义，用`|`将其与描述文字隔开(`<意图>|<描述>`)。

<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. 

如果所有地方出现的文本具有**相同**含义时，它们应该有**相同**的翻译，
但是如果在某些地方它具有**不同含义**，那么它应该有不同的翻译。
Angular的提取工具在翻译源文件中保留**含义**和**描述**，以支持符合特定上下文的翻译。


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

### 设置一个自定义的`id`来提升可搜索性和可维护性

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:

Angular 的 `i18n` 提取工具会为模板中每个带有`i18n`属性的元素生成一个*翻译单元（translation unit）*条目，并保存到一个文件中。默认情况下，它为每个翻译单元指定一个唯一的`id`，就像这样：

<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.

当我们修改这段可翻译的文字时，提取工具会为那个翻译单元生成一个新的`id`。
我们就要使用这个新的 id 来修改这个翻译文件。

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.

现在，提取工具和编译器就会用*你的自定义id`生成一个翻译单元，而不会再改变它。

<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:

下面这个例子带有*含义*和*描述*，最后是`id`：

<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.
  
为了确保定义出*唯一*的自定义id。如果我们对两个*不同的*文本块使用了同一个id，那么就只有一个会被提取出来，然后其翻译结果会被用于全部文本块。

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

### 翻译文本，而不必创建元素

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:

如果要翻译一段纯文本，我们就可以把它用`<span>`标签包裹起来。
但如果由于某些原因（比如CSS结构方面的考虑），我们可能不希望仅仅为了翻译而创建一个新的DOM元素，那么也可以把这段文本包裹进一个`<ng-container>`元素中。`<ng-container>`将被转换成一个HTML注释：

<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

## 添加 *i18n* 翻译属性

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

我们还可以翻译属性。
比如，假设我们的模板具有一个带`title`属性的图片：

<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.

这个 `title` 属性也需要翻译。

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.

我们也同样可以使用`i18n-x="<meaning>|<description>@@<id>"`语法来指定一个含义和描述。

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

## 处理单数与复数

Different languages have different pluralization rules.

不同的语言有不同的单复数规则。

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.

假设应用中需要谈论一些狼。
在英语中，根据狼的数量，可能要显示为"no wolves"、"one wolf"、"two wolves"或"a wolf pack"。
而在其它语言中则可能会有不同的**基数**规则。

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.

  第一个参数是key。它绑定到了组件中表示狼的数量的`wolves`属性。
  
* The second parameter identifies this as a `plural` translation type.

  第二个参数表示这是一个`plural`（复数）翻译类型。

* The third parameter defines a pluralization pattern consisting of pluralization categories and their matching values.
  
  第三个参数定义了一组复数表示模式，这个模式由复数类别和它们所匹配的值组成。

<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):

复数类别包括（取决于语言）：

* =0 (or any other number)

    =0 （或其它数字）
    
* zero

    zero（零）

* one

    one（一个)

* two

    two（两个）

* few

    few（少数）

* many

    many（很多）

* other

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

把默认的*英语*翻译结果放在复数类别之后的括号（`{}`）中。

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

## 在候选文本中选择

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:

组件模板中的下列消息格式绑定到了组件的`gender`属性，这个属性的取值是 "m" 或 "f" 或 "o"。
这个消息会把那些值映射到适当的翻译文本：

<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

## 把"复数"与"选择"表达式嵌套在一起

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

我们也可以把不同的 ICU 表达式嵌套在一起，比如：

<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_

## 使用_ng-xi18n_工具创建翻译源文件

Use the `ng xi18n` command provided by the CLI to extract the text messages marked with`i18n`into
 a translation source file .
 
 使用`ng-xi18n`提取工具来将带`i18n`标记的文本提取到一个翻译源文件中。


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

在应用的项目根目录打开一个终端窗口，并输入`ng-xi18n`命令：


<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

### 其它翻译格式

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:

我们可以使用`--i18nFormat`来明确指定想用的格式，范例如下：

<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

### 其它选项

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

我们还可能需要指定其它选项。
比如，如果TypeScript的配置文件`tsconfig.json`位于其它地方而不是根目录，我们就要通过`-p`选项来明确指出它的路径。

<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

## 翻译文本信息

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

`ng xi18n`命令在项目根目录生成一个名为`messages.xlf`的翻译源文件。

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.

下一步是将英文模板文本翻译到指定语言的翻译文件。
这个例子中创建了一个法语翻译文件。

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

### 新建一个本土化目录

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.

大多数应用都要被翻译成多种其它语言，因此，为全部国际化工作做适当的调整项目目录结构是一种标准实践。

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

其中一种方法是为本土化和相关资源（比如国际化文件）创建一个专门的目录。


<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>.

本土化和国际化是<a href="https://en.wikipedia.org/wiki/Internationalization_and_localization" target="_blank">不同但是很相近的概念</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

### 翻译文本节点

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

在现实世界中，`messages.es.xlf`文件会被发给西班牙语翻译，他们使用<a href="https://en.wikipedia.org/wiki/XLIFF#Editors" target="_blank">这些XLIFF文件编辑器</a>中的一种来翻译它。

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

我们不需要任何编辑器或者法语知识就可以轻易的翻译本例子文件。

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:

要翻译一个复数，就要翻译它的ICU格式中匹配的值：

<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_

### 翻译*选择*（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.

提取工具会把它拆成*两个*翻译单元，因为 ICU 表达式是分别提取的。

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.

第一个单元包含`select`之外的文本。
这里的`select`是一个占位符`<x id="ICU">`，用来表示`select`中的消息。
翻译这段文本，并把占位符放在那里。

<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.

第一个翻译单元的紧下方就是第二个翻译单元，包含`select`中的消息。翻译它。

<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:

在翻译之后，它们会放在一起：

<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

### 翻译嵌套的表达式

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:

嵌套的表达式和前一节没有什么不同。就像上一个例子中那样，我们有*两个*翻译单元。

<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:

第二个包含完整的嵌套表达式：

<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:

放在一起时：

<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

### 应用及其翻译文件

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

下面是例子应用及其翻译文件：


<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

## 合并已经翻译的文件

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.
  
    翻译文件
    
  * The translation file format.
  
    翻译文件的格式

  * 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 withthe `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

### 用JiT编译器合并

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

JiT（即时）编译器在应用程序加载时，在浏览器中编译应用。
在使用JiT编译器的环境中翻译是一个动态的流程，包括：

1. Importing the appropriate language translation file as a string constant.

  把合适的语言翻译文件导入成一个字符串常量
  
2. Creating corresponding translation providers for the JIT compiler.

  为JiT编译器创建相应的翻译提供商。
  
3. Bootstrapping the app with those providers.

  使用这些提供商来启动应用。

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

三种提供商帮助JiT编译在编译应用时，将模板文本翻译到某种语言：

* `TRANSLATIONS` is a string containing the content of the translation file.

    `TRANSLATIONS`是含有翻译文件内容的字符串。

* `TRANSLATIONS_FORMAT` is the format of the file: `xlf`, `xlf2`, or `xtb`.

    `TRANSLATIONS_FORMAT`是文件的格式: `xlf`、`xlif`或`xtb`。
    
* `LOCALE_ID` is the locale of the target language.

  `LOCALE_ID`是目标语言的语言环境。

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:

在下面的`src/app/i18n-providers.ts`文件的`getTranslationProviders()`函数中，根据用户的**语言环境**和对应的翻译文件构建这些提供商：


<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>