Site Settings: Add support for the language setting to the API (#8568)

* Add lang_id to the list of options available in the settings response
* Prevent modification & return an error code if the client cannot change the setting
* Download language packs if needed & available
* Let the `lang_id` param persist to the `WPLANG` option behind the scenes & control the setting
* Default to US english when option is empty
* Return an error code when the setting is "blocked" for the client.

_inc/lib/class.core-rest-api-endpoints.php

@@ -1889,6 +1889,13 @@ public static function get_updateable_data_list( $selector = '' ) {
 				'jp_group'          => 'settings',
 			),
 
+			'lang_id' => array(
+				'description' => esc_html__( 'Primary language for the site.', 'jetpack' ),
+				'type' => 'string',
+				'default' => 'en_US',
+				'jp_group' => 'settings',
+			),
+
 			'onboarding' => array(
 				'description'       => '',
 				'type'              => 'object',

_inc/lib/core-api/class.jetpack-core-api-module-endpoints.php

@@ -412,6 +412,23 @@ public function get_all_options() {
 
 		foreach ( $settings as $setting => $properties ) {
 			switch ( $setting ) {
+				case 'lang_id':
+					if ( defined( 'WPLANG' ) ) {
+						// We can't affect this setting, so warn the client
+						$response[ $setting ] = 'error_const';
+						break;
+					}
+
+					if ( ! current_user_can( 'install_languages' ) ) {
+						// The user doesn't have caps to install language packs, so warn the client
+						$response[ $setting ] = 'error_cap';
+						break;
+					}
+
+					$value = get_option( 'WPLANG' );
+					$response[ $setting ] = empty( $value ) ? 'en_US' : $value;
+					break;
+
 				case $holiday_snow_option_name:
 					$response[ $setting ] = get_option( $holiday_snow_option_name ) === 'letitsnow';
 					break;
@@ -607,6 +624,27 @@ public function update_data( $request ) {
 			$value = Jetpack_Core_Json_Api_Endpoints::cast_value( $value, $option_attrs );
 
 			switch ( $option ) {
+				case 'lang_id':
+					if ( defined( 'WPLANG' ) || ! current_user_can( 'install_languages' ) ) {
+						// We can't affect this setting
+						$updated = false;
+						break;
+					}
+
+					if ( $value === 'en_US' || empty( $value ) ) {
+						return delete_option( 'WPLANG' );
+					}
+
+					// `wp_download_language_pack` only tries to download packs if they're not already available
+					$language = wp_download_language_pack( $value );
+					if ( $language === false ) {
+						// The language pack download failed.
+						$updated = false;
+						break;
+					}
+					$updated = get_option( 'WPLANG' ) === $language ? true : update_option( 'WPLANG', $language );
+					break;
+
 				case 'monitor_receive_notifications':
 					$monitor = new Jetpack_Monitor();