My question is whatever I can login to ACP with DB PASSWORD if I fill the fields to login via Outh or there is a way to revert to DB in case the keys or CURL does not work ?
Regarding the extension at witch the information provided in the language file refers there is only for 3.1.x since probably their developer is on PHP 5.3.3

OneAll Social Login
Name:
oneall/sociallogin
Description:
Social Login for phpBB allows your users to login and register with 30+ Social Networks like for example Twitter, Facebook, LinkedIn and Google+
Version:
5.6.1
Page:
https://docs.oneall.com/plugins/guide/s ... gin-phpbb/
License:
GPL-2.0
https://www.phpbb.com/customise/db/mod/oa_social_login/