Kobe Wright
--- a/core/lib/Drupal/Core/Language/LanguageManager.php
+++ b/core/lib/Drupal/Core/Language/LanguageManager.php
    return $this->getDefaultLanguage();
  }

+  /**
+   * {@inheritdoc}
+   */
+  public function setCurrentLanguage(LanguageInterface $language, $type = LanguageInterface::TYPE_INTERFACE) {
--- a/core/lib/Drupal/Core/Language/LanguageManagerInterface.php
+++ b/core/lib/Drupal/Core/Language/LanguageManagerInterface.php

+  /**
+   * Sets the current language for the given type.
+   *
+   * @param \Drupal\Core\Language\LanguageInterface $language
+   *   The language to set as current language.
+   * @param string $type
+   *   (optional) The language type; e.g., the interface or the content
+   *   language. Defaults to
+   *   \Drupal\Core\Language\LanguageInterface::TYPE_INTERFACE.
+   *
+   * @return \Drupal\Core\Language\LanguageInterface
+   *   The current language object for the given type of language, before
+   *   setting the new current language.
+   */
+  public function setCurrentLanguage(LanguageInterface $language, $type = LanguageInterface::TYPE_INTERFACE);
--- a/core/modules/language/src/ConfigurableLanguageManager.php
+++ b/core/modules/language/src/ConfigurableLanguageManager.php
    return $this->negotiatedLanguages[$type];
  }

+  /**
+   * {@inheritdoc}
+   */
+  public function setCurrentLanguage(LanguageInterface $language, $type = LanguageInterface::TYPE_INTERFACE) {