Sometimes around the internet it happens. Somebody sets up a board real quick and doesn’t add the right language pack, setting it as standard. So when you want to change the standard board language in a phpBB3, it will not apply the feature before users are logged in and logged out again or they edit it manually.
UPDATE phpbb_users SET user_lang = "de" WHERE user_type = 2;
Your table doesn’t need to be called phpbb_users, but maybe has a prefix you defined yourself. Check in the settings of your board or with phpmyadmin. You should also check user_type and see the IDs available there so you can run the command for all groups in there.