[Docs] i18n asciidocs (#32547)

Context: #22349 

- [x] tell about locale
- [x] describe how to set a locale
- [x] describe how developers can use i18n engine functionality

docs/development/plugin-development.asciidoc

@@ -9,6 +9,7 @@ The Kibana plugin interfaces are in a state of constant development.  We cannot
 * <<development-plugin-resources>>
 * <<development-uiexports>>
 * <<development-plugin-functional-tests>>
+* <<development-plugin-localization>>
 * <<development-visualize-index>>
 * <<add-data-guide>>
 
@@ -19,6 +20,8 @@ include::plugin/development-uiexports.asciidoc[]
 
 include::plugin/development-plugin-functional-tests.asciidoc[]
 
+include::plugin/development-plugin-localization.asciidoc[]
+
 include::visualize/development-visualize-index.asciidoc[]
 
 include::add-data-guide.asciidoc[]

docs/development/plugin/development-plugin-localization.asciidoc

@@ -0,0 +1,164 @@
+[[development-plugin-localization]]
+=== Localization for plugins
+
+To introduce localization for your plugin, use our i18n tool to create IDs and default messages. You can then extract these IDs with respective default messages into localization JSON files for Kibana to use when running your plugin.
+
+[float]
+==== Adding localization to your plugin
+
+You must add a `translations` directory at the root of your plugin. This directory will contain the translation files that Kibana uses.
+
+["source","shell"]
+-----------
+.
+├── translations
+│   ├── en.json
+│   └── zh-CN.json
+└── .i18nrc.json
+-----------
+
+
+[float]
+==== Using Kibana i18n tooling
+To simplify the localization process, Kibana provides tools for the following functions:
+
+* Verify all translations have translatable strings and extract default messages from templates
+* Verify translation files and integrate them into Kibana
+
+To use Kibana i18n tooling, create a `.i18nrc.json` file with the following configs:
+
+* `paths`.  The directory from which the i18n translation IDs are extracted.
+* `exclude`. The list of files to exclude while parsing paths.
+* `translations`. The list of translations where JSON localizations are found.
+
+["source","json"]
+-----------
+{
+  "paths": {
+    "myPlugin": "src/ui",
+  },
+  "exclude": [
+  ],
+  "translations": [
+    "translations/zh-CN.json"
+  ]
+}
+-----------
+
+An example Kibana `.i18nrc.json` is {blob}.i18nrc.json[here].
+
+Full documentation about i18n tooling is {blob}src/dev/i18n/README.md[here].
+
+[float]
+==== Extracting default messages
+To extract the default messages from your plugin, run the following command:
+
+["source","shell"]
+-----------
+node scripts/i18n_extract --output-dir ./translations --include-config ../kibana-extra/myPlugin/.i18nrc.json
+-----------
+
+This outputs a `en.json` file inside the `translations` directory. To localize other languages, clone the file and translate each string.
+
+[float]
+==== Checking i18n messages
+
+Checking i18n does the following:
+
+* Checks all existing labels for violations.
+* Takes translations from .i18nrc.json and compares them to the messages extracted and validated at the step above and:
+** Checks for unused translations. If you remove a label that has a corresponding translation, you must also remove the label from the translations file.
+  * Checks for incompatible translations.  If you add or remove a new parameter from an existing string, you must also remove the label from the translations file.
+
+To check your i18n translations, run the following command:
+
+["source","shell"]
+-----------
+node scripts/i18n_check --fix --include-config ../kibana-extra/myPlugin/.i18nrc.json
+-----------
+
+
+[float]
+==== Implementing i18n in the UI
+
+Kibana relies on several UI frameworks (ReactJS and AngularJS) and
+requires localization in different environments (browser and NodeJS).
+The internationalization engine is framework agnostic and consumable in
+all parts of Kibana (ReactJS, AngularJS and NodeJS). In order to simplify
+internationalization in UI frameworks, additional abstractions are
+built around the I18n engine: `react-intl` for React and custom
+components for AngularJS. https://github.com/yahoo/react-intl[React-intl]
+is built around https://github.com/yahoo/intl-messageformat[intl-messageformat],
+so both React and AngularJS frameworks use the same engine and the same
+message syntax.
+
+
+[float]
+===== i18n for vanilla JavaScript
+
+["source","js"]
+-----------
+import { i18n } from '@kbn/i18n';
+
+export const HELLO_WORLD = i18n.translate('hello.wonderful.world', {
+  defaultMessage: 'Greetings, planet Earth!',
+});
+-----------
+
+Full details are {repo}tree/6.7/packages/kbn-i18n#vanilla-js[here].
+
+[float]
+===== i18n for React 
+
+To localize strings in React, use either `FormattedMessage` or `i18n.translate`.
+
+
+["source","js"]
+-----------
+import { i18n } from '@kbn/i18n';
+import { FormattedMessage } from '@kbn/i18n/react';
+
+export const Component = () => {
+  return (
+    <div>
+      {i18n.translate('xpack.someText', { defaultMessage: 'Some text' })}
+      <FormattedMessage id="xpack.someOtherText" defaultMessage="Some other text">
+      </FormattedMessage>
+    </div>
+  );
+};
+-----------
+
+Full details are {repo}tree/6.7/packages/kbn-i18n#react[here].
+
+
+
+[float]
+===== i18n for Angular 
+
+AngularJS wrapper has 4 entities: translation `provider`, `service`, `directive` and `filter`. Both the directive and the filter use the translation `service` with i18n engine under the hood.
+
+
+The translation directive has the following syntax:
+["source","js"]
+-----------
+<ANY
+  i18n-id="{string}"
+  i18n-default-message="{string}"
+  [i18n-values="{object}"]
+  [i18n-description="{string}"]
+></ANY>
+-----------
+
+Full details are {repo}tree/6.7/packages/kbn-i18n#angularjs[here].
+
+
+[float]
+==== Resources
+
+To learn more about i18n tooling, see {blob}src/dev/i18n/README.md[i18n dev tooling].
+
+To learn more about implementing i18n in the UI, follow the links below:
+
+* {blob}packages/kbn-i18n/README.md[i18n plugin]
+* {blob}packages/kbn-i18n/GUIDELINE.md[i18n Guidelines]

docs/settings/i18n-settings.asciidoc

@@ -0,0 +1,17 @@
+[role="xpack"]
+[[i18n-settings-kb]]
+=== i18n settings in Kibana
+
+You do not need to configure any settings to run Kibana in English.
+
+[float]
+[[general-i18n-settings-kb]]
+==== General i18n Settings
+
+`i18n.locale`::
+Kibana currently supports the following locales:
++
+- English - `en` (default)
+- Chinese - `zh-CN`
+
+

docs/settings/settings-xkb.asciidoc

@@ -18,3 +18,4 @@ include::logs-ui-settings.asciidoc[]
 include::ml-settings.asciidoc[]
 include::reporting-settings.asciidoc[]
 include::spaces-settings.asciidoc[]
+include::i18n-settings.asciidoc[]

docs/setup/settings.asciidoc

@@ -234,4 +234,6 @@ unauthenticated users to access the Kibana server status API and status page.
 
 `rollup.enabled:`:: *Default: true* Set this value to false to disable the Rollup user interface.
 
-`license_management.enabled`:: *Default: true* Set this value to false to disable the License Management user interface.
+`license_management.enabled`:: *Default: true* Set this value to false to disable the License Management user interface.
+
+`i18n.locale`:: *Default: en* Set this value to change the Kibana interface language. Valid locales are: `en`, `zh-CN`.