Index: sites/trunk/wordpress.org/public_html/wp-content/plugins/plugin-directory/api/routes/class-locale-banner.php
===================================================================
--- sites/trunk/wordpress.org/public_html/wp-content/plugins/plugin-directory/api/routes/class-locale-banner.php	(revision 4914)
+++ sites/trunk/wordpress.org/public_html/wp-content/plugins/plugin-directory/api/routes/class-locale-banner.php	(working copy)
@@ -68,7 +68,7 @@
 		$current_locale = get_locale();
 
 		// Build a list of WordPress locales which we'll suggest to the user.
-		$suggest_locales = array_values( array_intersect( $translated_locales, $locales_from_header ) );
+		$suggest_locales = array_values( array_intersect( $locales_from_header, $translated_locales ) );
 		$current_locale_is_suggested = in_array( $current_locale, $suggest_locales );
 		$current_locale_is_translated = in_array( $current_locale, $translated_locales );
 
@@ -262,8 +262,8 @@
 		}
 
 		foreach ( $available_locales as $locale ) {
-			list( $lang, ) = preg_split( '/[_-]/', $locale );
-			if ( $lang  ) {
+			list( $locale_lang, ) = preg_split( '/[_-]/', $locale );
+			if ( $lang === $locale_lang ) {
 				return $locale;
 			}
 		}
Index: sites/trunk/wordpress.org/public_html/wp-content/plugins/plugin-directory/tests/phpunit/tests/api/locale-banner.php
===================================================================
--- sites/trunk/wordpress.org/public_html/wp-content/plugins/plugin-directory/tests/phpunit/tests/api/locale-banner.php	(nonexistent)
+++ sites/trunk/wordpress.org/public_html/wp-content/plugins/plugin-directory/tests/phpunit/tests/api/locale-banner.php	(working copy)
@@ -0,0 +1,57 @@
+<?php
+
+/**
+ *
+ * @group api
+ */
+class Tests_API_Locale_Banner extends WP_UnitTestCase {
+
+	/**
+	 * @dataProvider data_locale_banner
+	 *
+	 * @type string $header         'Accept-Language' header value.
+	 * @type array  $expected_sites Rosetta sites that should be present in the banner text.
+	 */
+	function test_locale_banner( $header, $expected_sites ) {
+		$response = wp_remote_post( 'https://wordpress.org/plugins-wp/wp-json/plugins/v1/locale-banner', array(
+			'headers' => array(
+				'Accept-Language' => $header
+			),
+		) );
+
+		$data = json_decode( wp_remote_retrieve_body( $response ), true );
+
+		$this->assertArrayHasKey( 'suggest_string', $data );
+		$this->assertArrayHasKey( 'translated', $data );
+		$this->assertInternalType( 'array', $data['translated'] );
+
+		preg_match_all( '#[a-z-]+.wordpress.org#', $data['suggest_string'], $sites );
+
+		$this->assertNotEmpty( $sites );
+		$this->assertEquals( $expected_sites, $sites[0] );
+	}
+
+	/**
+	 * Data provider for test_locale_banner().
+	 *
+	 * @return array {
+	 *     @type array {
+	 *         @type string $header         'Accept-Language' header value.
+	 *         @type array  $expected_sites Rosetta sites that should be present in the banner text.
+	 *     }
+	 * }
+	 */
+	function data_locale_banner() {
+		return array(
+			array(
+				'en-US,en;q=0.8,ru;q=0.6,cs;q=0.4',
+				array( 'ru.wordpress.org', 'cs.wordpress.org' ),
+			),
+			array(
+				'en-US,en;q=0.8,da;q=0.6,nb;q=0.4,sv;q=0.2',
+				array( 'da.wordpress.org', 'nb.wordpress.org', 'sv.wordpress.org' ),
+			),
+		);
+	}
+
+}