In the English.xml file, some strings have different name and translation :

BUG: (CDATA) "Like" !== (name) "like"
BUG: (CDATA) "Unlike" !== (name) "unlike"
BUG: (CDATA) "Sticky" !== (name) "sticky"
BUG: (CDATA) "Unsticky" !== (name) "unsticky"
BUG: (CDATA) "Close" !== (name) "close"
BUG: (CDATA) "Open" !== (name) "open"
BUG: (CDATA) "Private" !== (name) "private"
BUG: (CDATA) "Public" !== (name) "public"
BUG: (CDATA) "Approved" !== (name) "approved"
BUG: (CDATA) "Unapproved" !== (name) "unapproved"
BUG: (CDATA) "Approve" !== (name) "approve"
BUG: (CDATA) "Recent Posts" !== (name) "Recently Added"

Would be cool to fix the location where they are used... so they can be the same in the XML file.

I also had to add these lines to the PO file:

#: wpf-includes/class-forums.php:50
msgid "Can approve/unapprove content"
msgstr ""

#: wpf-includes/class-forums.php:55
msgid "Can view attached files"
msgstr ""

#: wpf-includes/class-forums.php:60
msgid "Can create poll"
msgstr ""

#: wpf-includes/class-forums.php:61
msgid "Can vote poll"
msgstr ""

#: wpf-includes/class-forums.php:62
msgid "Can view poll result"
msgstr ""

#: wpf-admin/usergroup.php:145
msgid "Display on Members List"
msgstr ""

#: wpf-includes/class-usergroups.php:27
msgid "Dashboard - Moderation Menu"
msgstr ""

#: wpf-includes/class-usergroups.php:29
msgid "Dashboard - Can ban member"
msgstr ""

#: wpf-includes/class-usergroups.php:53
msgid "Front - Can view member occupation"
msgstr ""

#: wpf-includes/class-usergroups.php:54
msgid "Front - Can view member signature"
msgstr ""

#: wpf-includes/class-usergroups.php:32
msgid "Front - Can pass moderation"
msgstr ""

#: wpf-includes/class-usergroups.php:39
msgid "Front - Can have signature"
msgstr ""

#: wpf-includes/class-usergroups.php:35
msgid "Front - Can view member activity"
msgstr ""

#: wpf-includes/class-usergroups.php:32
msgid "Front - Can view member subscriptions"
msgstr ""

#: wpf-assets/js/frontend.js:164
msgid "Leave a comment"
msgstr ""

#: wpf-assets/js/frontend.js:140
msgid "Your answer"
msgstr ""

#: wpf-includes\class-members.php:1467
msgid "Length must be between 3 characters and 15 characters."
msgstr ""

#: 
msgid "Username length must be between 3 characters and 15 characters."
msgstr ""

#: wpf-includes\class-template.php:631
msgid "Recently Added"
msgstr ""

#: wpf-themes\classic\register.php:26
msgid "After registration you will receive an email confirmation with a link to set a new password"
msgstr ""

#: wpf-themes\classic\recent.php:92
msgid "View entire topic"
msgstr ""

#: wpf-includes\class-template.php:96 wpf-includes\class-template.php:242
msgid "Author Name"
msgstr ""

#: wpf-includes\class-template.php:97 wpf-includes\class-template.php:243
msgid "Your name"
msgstr ""

#: wpf-includes\class-template.php:100 wpf-includes\class-template.php:246
msgid "Author Email"
msgstr ""

#: wpf-includes\class-template.php:101 wpf-includes\class-template.php:247
msgid "Your email"
msgstr ""

#: wpf-includes\class-topics.php:166
msgid "Your topic successfully added and awaiting moderation"
msgstr ""

#: wpf-includes\class-topics.php:38
msgid "You don\t have permission to create topic into this forum"
msgstr ""

#: wpf-includes\class-posts.php:293
msgid "You don\t have permission to delete post from this forum"
msgstr ""

#: wpf-includes\class-topics.php:379
msgid "You don\t have permission to delete topic from this forum"
msgstr ""

#: wpf-includes\class-posts.php:220
msgid "You don\t have permission to edit post from this forum"
msgstr ""

The english.xml file have some lines which are missing single quote... they are also missing when these strings are used in the code (for example, in \wp-content\plugins\wpforo\wpf-includes\class-topics.php at line 38).

 <phrase name="You don\t have permission to create topic into this forum"><![CDATA[You don\t have permission to create topic into this forum]]></phrase>
 <phrase name="You don\t have permission to delete post from this forum"><![CDATA[You don\t have permission to delete post from this forum]]></phrase>
 <phrase name="You don\t have permission to delete topic from this forum"><![CDATA[You don\t have permission to delete topic from this forum]]></phrase>
 <phrase name="You don\t have permission to edit post from this forum"><![CDATA[You don\t have permission to edit post from this forum]]></phrase>

In english.xml, 2 lines are duplicates (they exists 2 times for the same "name" attributes):

<phrase name="New"><![CDATA[Nouveau]]></phrase>
<phrase name="unapproved"><![CDATA[non approuvé]]></phrase>

Discovered 2 more strings missing from the PO file:

#: wpf-assets/js/ajax.js:183
msgid "Reply with quote"
msgstr ""

#: wpf-assets/js/ajax.js:536
msgid "Edit post"
msgstr ""