[from https://www.phpbb.com/community/viewtopic.php?t=2642962:]

We had strange problems with phpBB concerning the language: At each first login, the language of the rendered page was English. Browser reload or visiting any other page solved the problem (i. e. the language became German, as intended).

Minimal reproducible example:

• Install de_x_sie (or I suppose any) language pack and activate that language.
• Create an auth plugin. Use the following code. 

  namespace your\namespace\auth\provider;

  class myauth extends \phpbb\auth\provider\base {

  public function autologin() {

  // the following code is taken from functions_user.php get_group_name :

  global $phpbb_container;

  $group_helper = $phpbb_container->get('group_helper')->get_name('foo');

  }

  public function login($username, $password) {}

  }

• Logout. Or clear the phpbb_sessions table.
• Visit the main page. Some strings like "Quick links" or "Delete cookies" ARE NOT translated.
• Reload the page. Now these strings ARE translated now.