diff --git a/src/js/_enqueues/admin/application-passwords.js b/src/js/_enqueues/admin/application-passwords.js
index 09e4434e2f..b1a20ceb8e 100644
--- a/src/js/_enqueues/admin/application-passwords.js
+++ b/src/js/_enqueues/admin/application-passwords.js
@@ -47,7 +47,7 @@
 		request = wp.hooks.applyFilters( 'wp_application_passwords_new_password_request', request, userId );
 
 		wp.apiRequest( {
-			path: '/wp/v2/users/' + userId + '/application-passwords',
+			path: '/wp/v2/users/' + userId + '/application-passwords?_locale=user',
 			method: 'POST',
 			data: request
 		} ).always( function() {
@@ -94,7 +94,7 @@
 		$submitButton.prop( 'disabled', true );
 
 		wp.apiRequest( {
-			path: '/wp/v2/users/' + userId + '/application-passwords/' + uuid,
+			path: '/wp/v2/users/' + userId + '/application-passwords/' + uuid + '?_locale=user',
 			method: 'DELETE'
 		} ).always( function() {
 			$submitButton.prop( 'disabled', false );
@@ -123,7 +123,7 @@
 		$submitButton.prop( 'disabled', true );
 
 		wp.apiRequest( {
-			path: '/wp/v2/users/' + userId + '/application-passwords',
+			path: '/wp/v2/users/' + userId + '/application-passwords?_locale=user',
 			method: 'DELETE'
 		} ).always( function() {
 			$submitButton.prop( 'disabled', false );
diff --git a/src/js/_enqueues/admin/auth-app.js b/src/js/_enqueues/admin/auth-app.js
index 4af3a05fa8..ad4403032d 100644
--- a/src/js/_enqueues/admin/auth-app.js
+++ b/src/js/_enqueues/admin/auth-app.js
@@ -54,7 +54,7 @@
 		request = wp.hooks.applyFilters( 'wp_application_passwords_approve_app_request', request, context );
 
 		wp.apiRequest( {
-			path: '/wp/v2/users/me/application-passwords',
+			path: '/wp/v2/users/me/application-passwords?_locale=user',
 			method: 'POST',
 			data: request
 		} ).done( function( response, textStatus, jqXHR ) {
diff --git a/src/js/_enqueues/admin/site-health.js b/src/js/_enqueues/admin/site-health.js
index d91b75e90a..9bba750094 100644
--- a/src/js/_enqueues/admin/site-health.js
+++ b/src/js/_enqueues/admin/site-health.js
@@ -271,7 +271,7 @@ jQuery( document ).ready( function( $ ) {
 
 				if ( 'undefined' !== typeof( this.has_rest ) && this.has_rest ) {
 					wp.apiRequest( {
-						url: this.test,
+						url: wp.url.addQueryArgs( this.test, { _locale: 'user' } ),
 						headers: this.headers
 					} )
 						.done( function( response ) {
diff --git a/src/js/_enqueues/wp/api-request.js b/src/js/_enqueues/wp/api-request.js
index 885e5b47f8..58ef531179 100644
--- a/src/js/_enqueues/wp/api-request.js
+++ b/src/js/_enqueues/wp/api-request.js
@@ -10,6 +10,7 @@
  *
  * @since 4.9.0
  * @since 5.6.0 Added overriding of the "PUT" and "DELETE" methods with "POST".
+ *              Added an "application/json" Accept header to all requests.
  * @output wp-includes/js/api-request.js
  */
 
@@ -26,7 +27,7 @@
 		var path = options.path;
 		var method = options.method;
 		var namespaceTrimmed, endpointTrimmed, apiRoot;
-		var headers, addNonceHeader, headerName;
+		var headers, addNonceHeader, addAcceptHeader, headerName;
 
 		if (
 			typeof options.namespace === 'string' &&
@@ -55,19 +56,24 @@
 
 		// If ?_wpnonce=... is present, no need to add a nonce header.
 		addNonceHeader = ! ( options.data && options.data._wpnonce );
+		addAcceptHeader = true;
 
 		headers = options.headers || {};
 
-		// If an 'X-WP-Nonce' header (or any case-insensitive variation
-		// thereof) was specified, no need to add a nonce header.
-		if ( addNonceHeader ) {
-			for ( headerName in headers ) {
-				if ( headers.hasOwnProperty( headerName ) ) {
-					if ( headerName.toLowerCase() === 'x-wp-nonce' ) {
-						addNonceHeader = false;
-						break;
-					}
-				}
+		for ( headerName in headers ) {
+			if ( ! headers.hasOwnProperty( headerName ) ) {
+				continue;
+			}
+
+			// If an 'X-WP-Nonce' or 'Accept' header (or any case-insensitive variation
+			// thereof) was specified, no need to add the header again.
+			switch ( headerName.toLowerCase() ) {
+				case 'x-wp-nonce':
+					addNonceHeader = false;
+					break;
+				case 'accept':
+					addAcceptHeader = false;
+					break;
 			}
 		}
 
@@ -78,6 +84,12 @@
 			}, headers );
 		}
 
+		if ( addAcceptHeader ) {
+			headers = $.extend( {
+				'Accept': 'application/json, */*;q=0.1'
+			}, headers );
+		}
+
 		if ( typeof method === 'string' ) {
 			method = method.toUpperCase();
 
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-site-health-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-site-health-controller.php
index 6dbc239ec0..f790dadb04 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-site-health-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-site-health-controller.php
@@ -171,6 +171,7 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 	 * @return array
 	 */
 	public function test_background_updates() {
+		$this->load_admin_textdomain();
 		return $this->site_health->get_test_background_updates();
 	}
 
@@ -182,6 +183,7 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 	 * @return array
 	 */
 	public function test_dotorg_communication() {
+		$this->load_admin_textdomain();
 		return $this->site_health->get_test_dotorg_communication();
 	}
 
@@ -193,6 +195,7 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 	 * @return array
 	 */
 	public function test_loopback_requests() {
+		$this->load_admin_textdomain();
 		return $this->site_health->get_test_loopback_requests();
 	}
 
@@ -204,6 +207,7 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 	 * @return array
 	 */
 	public function test_authorization_header() {
+		$this->load_admin_textdomain();
 		return $this->site_health->get_test_authorization_header();
 	}
 
@@ -219,6 +223,8 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 			require_once ABSPATH . 'wp-admin/includes/class-wp-debug-data.php';
 		}
 
+		$this->load_admin_textdomain();
+
 		$sizes_data = WP_Debug_Data::get_sizes();
 		$all_sizes  = array( 'raw' => 0 );
 
@@ -256,6 +262,22 @@ class WP_REST_Site_Health_Controller extends WP_REST_Controller {
 		return $all_sizes;
 	}
 
+	/**
+	 * Loads the admin textdomain for Site Health tests.
+	 *
+	 * The {@see WP_Site_Health} class is defined in WP-Admin, while the REST API operates in a front-end context.
+	 * This means that the translations for Site Health won't be loaded by default in {@see load_default_textdomain()}.
+	 *
+	 * @since 5.6.0
+	 */
+	protected function load_admin_textdomain() {
+		// Accounts for inner REST API requests in the admin.
+		if ( ! is_admin() ) {
+			$locale = determine_locale();
+			load_textdomain( 'default', WP_LANG_DIR . "/admin-$locale.mo" );
+		}
+	}
+
 	/**
 	 * Gets the schema for each site health test.
 	 *
diff --git a/src/wp-includes/script-loader.php b/src/wp-includes/script-loader.php
index f3ad2de05c..a0da5a0c4d 100644
--- a/src/wp-includes/script-loader.php
+++ b/src/wp-includes/script-loader.php
@@ -1286,7 +1286,7 @@ function wp_default_scripts( $scripts ) {
 		$scripts->add( 'plugin-install', "/wp-admin/js/plugin-install$suffix.js", array( 'jquery', 'jquery-ui-core', 'thickbox' ), false, 1 );
 		$scripts->set_translations( 'plugin-install' );
 
-		$scripts->add( 'site-health', "/wp-admin/js/site-health$suffix.js", array( 'clipboard', 'jquery', 'wp-util', 'wp-a11y', 'wp-api-request' ), false, 1 );
+		$scripts->add( 'site-health', "/wp-admin/js/site-health$suffix.js", array( 'clipboard', 'jquery', 'wp-util', 'wp-a11y', 'wp-api-request', 'wp-url' ), false, 1 );
 		$scripts->set_translations( 'site-health' );
 
 		$scripts->add( 'privacy-tools', "/wp-admin/js/privacy-tools$suffix.js", array( 'jquery', 'wp-a11y' ), false, 1 );
diff --git a/tests/qunit/wp-includes/js/api-request.js b/tests/qunit/wp-includes/js/api-request.js
index 86d0d72cda..aa7ee1dea7 100644
--- a/tests/qunit/wp-includes/js/api-request.js
+++ b/tests/qunit/wp-includes/js/api-request.js
@@ -2,7 +2,10 @@
 ( function( QUnit ) {
 	var originalRootUrl = window.wpApiSettings.root;
 
-	var nonceHeader = { 'X-WP-Nonce': 'not_a_real_nonce' };
+	var expectedHeaders = {
+		'X-WP-Nonce': 'not_a_real_nonce',
+		'Accept': 'application/json, */*;q=0.1'
+	};
 
 	QUnit.module( 'wp-api-request', {
 		afterEach: function() {
@@ -32,6 +35,7 @@
 			url: 'http://localhost/wp-json/wp/v2/posts',
 			headers: {
 				'X-WP-Nonce': 'not_a_real_nonce',
+				'Accept': 'application/json, */*;q=0.1',
 				'Header-Name': 'value'
 			},
 			data: {
@@ -64,11 +68,15 @@
 			var settings = wp.apiRequest.buildAjaxOptions( settingsOriginal );
 
 			assert.notStrictEqual( settings, settingsOriginal );
-			assert.strictEqual( settings.headers, settingsOriginal.headers );
+
+			var expected = {
+				Accept: 'application/json, */*;q=0.1'
+			};
+			expected[ headerName ] = nonceHeader[ headerName ];
 
 			assert.deepEqual( settings, {
 				url: 'aaaa',
-				headers: nonceHeader
+				headers: expected
 			} );
 		} );
 	} );
@@ -87,20 +95,35 @@
 
 		assert.deepEqual( settings, {
 			url: 'aaaa',
-			headers: {},
+			headers: {
+				'Accept': 'application/json, */*;q=0.1'
+			},
 			data: {
 				_wpnonce: 'definitely_not_a_real_nonce'
 			}
 		} );
 	} );
 
+	QUnit.test( 'does not add accept header if already present', function( assert ) {
+		var settingsOriginal = {
+			url: 'aaaa',
+			headers: {
+				'Accept': 'text/xml'
+			}
+		};
+
+		var settings = wp.apiRequest.buildAjaxOptions( settingsOriginal );
+
+		assert.strictEqual( settingsOriginal.headers.Accept, settings.headers.Accept );
+	} );
+
 	QUnit.test( 'accepts namespace and endpoint', function( assert ) {
 		assert.deepEqual( wp.apiRequest.buildAjaxOptions( {
 			namespace: 'wp/v2',
 			endpoint: 'posts'
 		} ), {
 			url: 'http://localhost/wp-json/wp/v2/posts',
-			headers: nonceHeader
+			headers: expectedHeaders
 		} );
 	} );
 
@@ -110,7 +133,7 @@
 			endpoint: '/posts'
 		} ), {
 			url: 'http://localhost/wp-json/wp/v2/posts',
-			headers: nonceHeader
+			headers: expectedHeaders
 		} );
 	} );
 
@@ -120,7 +143,7 @@
 			endpoint: ''
 		} ), {
 			url: 'http://localhost/wp-json/wp/v2',
-			headers: nonceHeader
+			headers: expectedHeaders
 		} );
 	} );
 
@@ -130,7 +153,7 @@
 			endpoint: ''
 		} ), {
 			url: 'http://localhost/wp-json/',
-			headers: nonceHeader
+			headers: expectedHeaders
 		} );
 	} );
 
@@ -143,7 +166,7 @@
 				endpoint: '/posts?orderby=title'
 			} ), {
 				url: 'http://localhost/index.php?rest_route=/wp/v2/posts&orderby=title',
-				headers: nonceHeader
+				headers: expectedHeaders
 			} );
 		}
 	);
@@ -157,7 +180,7 @@
 				endpoint: ''
 			} ), {
 				url: 'http://localhost/index.php?rest_route=/',
-				headers: nonceHeader
+				headers: expectedHeaders
 			} );
 		}
 	);