group Wp and group PHPBB

As a result, the creator account of the forum and the creator account of the site are no longer administrators or installers accounts which is problematic for your installation procedure

To install without problems, you only should do this:
setup an username in WP, and another, equal, in phpBB, both with same email.
So activate the plugin using this user.
This is not a problem.

Is it possible that the accounts that are administrator / moderators in PHPBB are then administrator / editor in WP?

Yes i will fix with a patch within tomorrow my time, the 1.7.8, than i will may reply here: it should not, but at moment the plugin add all users as subscribers.
I’ve wait to answer to this question, because i want re-test this thing: as said, it should not, but instead add all users as subscribers.
You can download a temp working patched version of class.wp.w3all-phpbb.zip here:
https://www.axew3.com/common/class.wp.w3all-phpbb.zip
this will add a coming logging in WP, or logged phpBB user, into WP, as:
administrator, if the user belong as primary group to ADMINISTRATORS into phpBB
editor, if the user belong as primary group to GLOBAL_MODERATORS into phpBB

Yes it is possible to add with easy any further custom group if necessary, but it need to be added in case.
(this patch not affect manual transfer phpBB to WP users, that is not mandatory, affect onlogin users in WP side, or phpBB logged users coming in WP).

When the patch will applied, the code will be changed on next 1.7.9 in favor of the native way of WP to add users with specified role, that seem so not work in the way it is on 1.7.9 (and from long time i realize). On 1.7.9 will be added also to the not mandatory phpBB to WP users transfer process.

Note that as on procedure help, it is mandatory instead, when integration start, transfer old WP users into phpBB.

About language: very nice!
the not so easy easy way, is here (read on top about Translator Handbook link):
https://translate.ww.wp.xz.cn/projects/wp-plugins/wp-w3all-phpbb-integration
french is one of th most well translated!
By the way, the needed translations for frontend, are really few! 10-12 strings or less if i’m not wrong.

1.7.8 has been patched about this and another bug:
https://ww.wp.xz.cn/support/topic/1-7-8-patches-log/#post-9708021
you should replace the class.wp.w3all-phpbb.php file with the new one patched:
https://plugins.trac.ww.wp.xz.cn/export/HEAD/wp-w3all-phpbb-integration/trunk/class.wp.w3all-phpbb.php
that contain correct fixes.

The patch to the other file wp_w3all_users_to_wp.php about (not mandatory) transfer procedure phpBB users to WP, if needed has been also patched.