How to change title of "Log in" link in account menu for anonymous users?
As described by @Matt in the answer the menu link is provided by the user module and you can simply decide not to use it and build your own links.

The link in the user module is a dynamic link where a menu link plugin  generates a different title depending on whether the user is authenticated or not.

To change the title you can replace this plugin class:

/**
 * Implements hook_menu_links_discovered_alter().
 */
function mymodule_menu_links_discovered_alter(&amp;$links) {
  $links['user.logout']['class'] = 'Drupal\mymodule\Plugin\Menu\MyLoginLogoutMenuLink';
}


and override the method getTitle() with your own content:

mymodule/src/Plugin/Menu/MyLoginLogoutMenuLink.php:

&lt;?php

namespace Drupal\mymodule\Plugin\Menu;

use Drupal\user\Plugin\Menu\LoginLogoutMenuLink;

class MyLoginLogoutMenuLink extends LoginLogoutMenuLink {

  public function getTitle() {
    if ($this-&gt;currentUser-&gt;isAuthenticated()) {
      return $this-&gt;t('My Log out');
    }
    else {
      return $this-&gt;t('My Log in');
    }
  }

}


A third option would be to change the title in an English to English translation, see How do I translate to English?