diff --git a/tests/src/Functional/ExistingSiteTestBase.php b/tests/src/Functional/ExistingSiteTestBase.php
new file mode 100644
index 00000000..31a9a827
--- /dev/null
+++ b/tests/src/Functional/ExistingSiteTestBase.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\Tests\helfi_api_base\Functional;
+
+use Drupal\Tests\helfi_api_base\Traits\DefaultConfigurationTrait;
+use weitzman\DrupalTestTraits\ExistingSiteBase;
+
+/**
+ * Existing site test base.
+ */
+abstract class ExistingSiteTestBase extends ExistingSiteBase {
+
+  use DefaultConfigurationTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() : void {
+    parent::setUp();
+
+    $this->setupDefaultConfiguration();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function tearDown() : void {
+    parent::tearDown();
+    $this->tearDownDefaultConfiguration();
+  }
+
+}
diff --git a/tests/src/FunctionalJavascript/ExistingSiteJavascriptTestBase.php b/tests/src/FunctionalJavascript/ExistingSiteJavascriptTestBase.php
new file mode 100644
index 00000000..c31fec53
--- /dev/null
+++ b/tests/src/FunctionalJavascript/ExistingSiteJavascriptTestBase.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\Tests\helfi_api_base\FunctionalJavascript;
+
+use Drupal\Tests\helfi_api_base\Traits\DefaultConfigurationTrait;
+use weitzman\DrupalTestTraits\ExistingSiteSelenium2DriverTestBase;
+
+/**
+ * Existing site test base.
+ */
+abstract class ExistingSiteJavascriptTestBase extends ExistingSiteSelenium2DriverTestBase {
+
+  use DefaultConfigurationTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() : void {
+    parent::setUp();
+
+    $this->setupDefaultConfiguration();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function tearDown() : void {
+    parent::tearDown();
+    $this->tearDownDefaultConfiguration();
+  }
+
+}
diff --git a/tests/src/Traits/DefaultConfigurationTrait.php b/tests/src/Traits/DefaultConfigurationTrait.php
new file mode 100644
index 00000000..54375654
--- /dev/null
+++ b/tests/src/Traits/DefaultConfigurationTrait.php
@@ -0,0 +1,92 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Drupal\Tests\helfi_api_base\Traits;
+
+use Drupal\Core\Config\Config;
+use Drupal\Core\Language\LanguageInterface;
+use Drupal\Core\Url;
+
+/**
+ * Basic configuration for each test.
+ */
+trait DefaultConfigurationTrait {
+
+  /**
+   * The default language.
+   *
+   * @var string
+   */
+  protected string $defaultLanguage = '';
+
+  /**
+   * Gets the language object for given langcode.
+   *
+   * @param string $langcode
+   *   The langcode.
+   *
+   * @return \Drupal\Core\Language\LanguageInterface
+   *   The language.
+   */
+  protected function getLanguage(string $langcode) : LanguageInterface {
+    return \Drupal::languageManager()->getLanguage($langcode);
+  }
+
+  /**
+   * Wrapper for drupalGet() to always set language code.
+   *
+   * @param string|\Drupal\Core\Url $url
+   *   The url.
+   * @param string $langcode
+   *   The langcode.
+   * @param array $options
+   *   The options.
+   * @param array $headers
+   *   The headers.
+   */
+  protected function drupalGetWithLanguage(string|Url $url, string $langcode = 'en', array $options = [], array $headers = []) : void {
+    $options['language'] = $this->getLanguage($langcode);
+    $this->drupalGet($url, $options, $headers);
+  }
+
+  /**
+   * Set up the default configuration.
+   */
+  protected function setupDefaultConfiguration() : void {
+    $this->defaultLanguage = $this->getDefaultLanguageConfiguration()
+      ->get('selected_langcode');
+    $this->setDefaultLanguage('en');
+  }
+
+  /**
+   * Restores the default configuration.
+   */
+  protected function tearDownDefaultConfiguration() : void {
+    $this->setDefaultLanguage($this->defaultLanguage);
+  }
+
+  /**
+   * Gets the configuration.
+   *
+   * @return \Drupal\Core\Config\Config
+   *   The default configuration.
+   */
+  protected function getDefaultLanguageConfiguration() : Config {
+    return \Drupal::configFactory()
+      ->getEditable('language.negotiation');
+  }
+
+  /**
+   * Sets the default language.
+   *
+   * @param string $langcode
+   *   The langcode to set as default.
+   */
+  protected function setDefaultLanguage(string $langcode) : void {
+    $this->getDefaultLanguageConfiguration()
+      ->set('selected_langcode', $langcode)
+      ->save();
+  }
+
+}