diff --git a/src/Form/LanguageTagSettingsForm.php b/src/Form/LanguageTagSettingsForm.php
index 2659376..cfbac96 100644
--- a/src/Form/LanguageTagSettingsForm.php
+++ b/src/Form/LanguageTagSettingsForm.php
@@ -116,27 +116,27 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
   /**
    * Prepares the submitted value to be stored in config.
    *
-   * @param string $value
+   * @param string|null $value
    *   The submitted value.
    *
    * @return array
    *   The value to be stored in config.
    */
-  public static function multiLineStringToArray(string $value): array {
-    return array_filter(array_map('trim', explode("\n", trim($value))));
+  public static function multiLineStringToArray(?string $value): array {
+    return array_filter(array_map('trim', explode("\n", trim($value ?? ''))));
   }
 
   /**
    * Prepares the config value to be displayed in the form.
    *
-   * @param array $value
+   * @param array|null $value
    *   The value saved in config.
    *
    * @return string
    *   The value of the form element.
    */
-  public static function arrayToMultiLineString(array $value): string {
-    return implode("\n", $value);
+  public static function arrayToMultiLineString(?array $value): string {
+    return implode("\n", $value ?? []);
   }
 
 }
diff --git a/tests/src/Functional/LanguageTagSettingsFormTest.php b/tests/src/Functional/LanguageTagSettingsFormTest.php
new file mode 100644
index 0000000..d067168
--- /dev/null
+++ b/tests/src/Functional/LanguageTagSettingsFormTest.php
@@ -0,0 +1,198 @@
+<?php
+
+namespace Drupal\Tests\ewp_core\Functional;
+
+use Drupal\user\Entity\User;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the language settings form.
+ *
+ * @group ewp_core
+ */
+class LanguageTagSettingsFormTest extends BrowserTestBase {
+
+  const CONFIG_KEY = 'ewp_core.settings';
+  const FORM_PATH = 'admin/ewp/settings/lang';
+  const PERMISSION = 'administer ewp lang settings';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['user', 'ewp_core'];
+
+  /**
+   * Admin user account.
+   */
+  protected User $admin;
+
+  /**
+   * Authenticated user account.
+   */
+  protected User $user;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $this->user = $this->drupalCreateUser(['access content']);
+    $this->admin = $this->drupalCreateUser([self::PERMISSION]);
+  }
+
+  /**
+   * Tests access to the language tag settings form by a non-privileged user.
+   */
+  public function testLanguageTagSettingsFormWithoutPermission() {
+    $this->drupalLogin($this->user);
+
+    $this->drupalGet(self::FORM_PATH);
+    $this->assertSession()
+      ->statusCodeEquals(403);
+  }
+
+  /**
+   * Tests the language tag settings form as a privileged user.
+   */
+  public function testLanguageTagSettingsForm() {
+    $this->drupalLogin($this->admin);
+
+    // Test the default configuration.
+    $default_config = $this->config(self::CONFIG_KEY);
+
+    $lang_primary_group_label = $default_config->get('lang_primary_group_label');
+    $this->assertEquals('EU official languages', $lang_primary_group_label);
+
+    $lang_primary_list = $default_config->get('lang_primary_list');
+    $this->assertContains('el|Greek', $lang_primary_list);
+
+    $lang_secondary_group_label = $default_config->get('lang_secondary_group_label');
+    $this->assertEquals('Other languages', $lang_secondary_group_label);
+
+    $lang_secondary_list = $default_config->get('lang_secondary_list');
+    $this->assertContains('tr|Turkish', $lang_secondary_list);
+
+    // Test access to the language tag settings form.
+    $this->drupalGet(self::FORM_PATH);
+    $this->assertSession()
+      ->statusCodeEquals(200);
+
+    // Test default values on the page.
+    $this->assertSession()
+      ->pageTextMatchesCount(2, '/Greek/');
+    $this->assertSession()
+      ->pageTextMatchesCount(1, '/Turkish/');
+
+    // Test form submission.
+    $submission_data = [
+      'lang_primary_group_label' => 'European',
+      'lang_primary_list' => implode("\n", [
+        'pt-PT|Portuguese (Portugal)',
+        'es-ES|Spanish (Spain)',
+      ]),
+      'lang_secondary_group_label' => 'South American',
+      'lang_secondary_list' => implode("\n", [
+        'pt-BR|Portuguese (Brazil)',
+        'es-AR|Spanish (Argentina)',
+      ]),
+    ];
+    $this->submitForm($submission_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains('The configuration options have been saved.');
+
+    // Test the configuration has been updated.
+    $new_config = $this->config(self::CONFIG_KEY);
+
+    $lang_primary_group_label = $new_config->get('lang_primary_group_label');
+    $this->assertEquals('European', $lang_primary_group_label);
+
+    $lang_primary_list = $new_config->get('lang_primary_list');
+    $this->assertContains('pt-PT|Portuguese (Portugal)', $lang_primary_list);
+    $this->assertNotContains('el|Greek', $lang_primary_list);
+
+    $lang_secondary_group_label = $new_config->get('lang_secondary_group_label');
+    $this->assertEquals('South American', $lang_secondary_group_label);
+
+    $lang_secondary_list = $new_config->get('lang_secondary_list');
+    $this->assertContains('pt-BR|Portuguese (Brazil)', $lang_secondary_list);
+    $this->assertNotContains('tr|Turkish', $lang_secondary_list);
+
+    // Test the form loads normally.
+    $this->drupalGet(self::FORM_PATH);
+    $this->assertSession()
+      ->statusCodeEquals(200);
+
+    // Test new values on the page.
+    $this->assertSession()
+      ->pageTextMatchesCount(2, '/Portuguese/');
+    $this->assertSession()
+      ->pageTextMatchesCount(2, '/Spanish/');
+
+  }
+
+  /**
+   * Tests incorrect inputs in the language tag settings form.
+   */
+  public function testLanguageTagSettingsFormErrors() {
+    $this->drupalLogin($this->admin);
+
+    $this->drupalGet(self::FORM_PATH);
+
+    // Some correct data to begin with.
+    $correct_data = [
+      'lang_primary_group_label' => 'Primary',
+      'lang_primary_list' => 'en|English',
+      'lang_secondary_group_label' => 'Secondary',
+      'lang_secondary_list' => 'und|undefined',
+    ];
+    $this->submitForm($correct_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains('The configuration options have been saved.');
+
+    // Test values cannot be empty.
+    $bad_data = $correct_data;
+    $bad_data['lang_primary_group_label'] = '';
+    $this->submitForm($bad_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains('This value should not be blank.');
+
+    $bad_data = $correct_data;
+    $bad_data['lang_primary_list'] = '';
+    $this->submitForm($bad_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains('This value should not be blank.');
+
+    $bad_data = $correct_data;
+    $bad_data['lang_secondary_group_label'] = '';
+    $this->submitForm($bad_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains('This value should not be blank.');
+
+    $bad_data = $correct_data;
+    $bad_data['lang_secondary_list'] = '';
+    $this->submitForm($bad_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains('This value should not be blank.');
+
+    // Test language tags must be valid.
+    $bad_data = $correct_data;
+    $bad_data['lang_primary_list'] = 'xyz';
+    $this->submitForm($bad_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains("'xyz' is not a valid language tag.");
+
+    $bad_data = $correct_data;
+    $bad_data['lang_secondary_list'] = 'pt-Madeira';
+    $this->submitForm($bad_data, 'Save configuration');
+    $this->assertSession()
+      ->pageTextContains("'pt-Madeira' is not a valid language tag.");
+
+  }
+
+}