Regarding the JS/Ajax notifications (the ones using WPF()->notice->add() ), a lot of them are not translated in the PO file.

  0 => string 'User registration is disabled.' (length=30)
  1 => string 'Username length must be between %d characters and %d characters.' (length=64)
  2 => string 'ERROR: invalid_username. Sorry, that username is not allowed. Please insert another.' (length=84)
  3 => string 'Password length must be between %d characters and %d characters.' (length=64)
  5 => string 'Success! Please check your mail for confirmation.' (length=49)
  6 => string 'Numerical nicknames are not allowed. Please insert another.' (length=59)
  7 => string 'This nickname is already registered. Please insert another.' (length=59)
  8 => string 'Userid is wrong' (length=15)
  9 => string 'Password successfully changed' (length=29)
  10 => string 'Avatar image is too big maximum allowed size is 2MB' (length=51)
  11 => string 'You can't make yourself banned user' (length=35)
  12 => string 'User successfully banned from wpforo' (length=36)
  13 => string 'User ban action error' (length=21)
  14 => string 'User successfully unbanned from wpforo' (length=38)
  15 => string 'User unban action error' (length=23)
  16 => string 'error: Change Status action' (length=27)
  18 => string 'API options successfully updated' (length=32)
  19 => string 'API options successfully updated, but previous value not changed' (length=64)
  20 => string 'Deleted' (length=7)
  22 => string 'action error' (length=12)
  23 => string 'post not found' (length=14)
  24 => string 'You don\t have permission to like posts from this forum' (length=55)
  25 => string 'done' (length=4)
  29 => string 'topic not found' (length=15)
  30 => string 'You don\t have permission to make topic answered' (length=48)
  32 => string 'Error: please insert some text to report.' (length=41)
  33 => string 'The key is expired' (length=18)
  34 => string 'The key is invalid' (length=18)
  35 => string 'Email has been sent' (length=19)
  38 => string 'The password reset mismatch' (length=27)
  39 => string 'The password reset empty' (length=24)
  40 => string 'The password has been changed' (length=29)
  41 => string 'Invalid request.' (length=16)

Thank you for letting us know, we'll add those in POT file.

 Here is the strings I added to the PO file regarding the notifications:

For now, I will add all the PO strings to your phrases system because I have no idea which one are used in the front end and are missing from the phrases system. Because if they are not in your phrase system, they can't be exported to JS. 

I attach an empty PO file with all the added strings (at least the one that can be translated somehow): http://www.crealya.fr/wpforo/wpforo-fr_FR-empty.zip

#: 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 ""

#: wpf-includes\wpf-phrases.php:572
msgid "Can edit own replay"
msgstr ""

#: wpf-includes\wpf-phrases.php:574
msgid "Can delete own replay"
msgstr ""

#: wpf-includes\class-forums.php:54 wpf-includes\wpf-phrases.php:580
msgid "Can Attach File"
msgstr ""

#: wpf-includes\wpf-phrases.php:697
msgid "Members Search Type:"
msgstr ""

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

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

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

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

#: wpf-includes\class-members.php:209
msgid "User registration is disabled."
msgstr ""

#: wpf-includes\class-members.php:249 wpf-includes\class-members.php:300
msgid "Username length must be between %d characters and %d characters."
msgstr ""

#: wpf-includes\class-members.php:257
msgid "ERROR: invalid_username. Sorry, that username is not allowed. Please insert another."
msgstr ""

#: wpf-includes\class-members.php:273
msgid "Password length must be between %d characters and %d characters."
msgstr ""

#: wpf-themes\classic\profile-account.php:34 wpf-includes\class-members.php:307
msgid "Success! Please check your mail for confirmation."
msgstr ""

#: wpf-includes\class-members.php:366
msgid "Numerical nicknames are not allowed. Please insert another."
msgstr ""

#: wpf-includes\class-members.php:371
msgid "This nickname is already registered. Please insert another."
msgstr ""

#: wpf-includes\class-members.php:413
msgid "Userid is wrong"
msgstr ""

#: wpf-includes\class-members.php:429
msgid "Password successfully changed"
msgstr ""

#: wpf-includes\class-members.php:459
msgid "Avatar image is too big maximum allowed size is 2MB"
msgstr ""

#: wpf-includes\class-members.php:733
msgid "You can't make yourself banned user"
msgstr ""

#: wpf-includes\class-members.php:749
msgid "User successfully banned from wpforo"
msgstr ""

#: wpf-includes\class-members.php:753
msgid "User ban action error"
msgstr ""

#: wpf-includes\class-members.php:771
msgid "User successfully unbanned from wpforo"
msgstr ""

#: wpf-includes\class-members.php:775
msgid "User unban action error"
msgstr ""

#: wpf-includes\class-topics.php:926 wpf-includes\class-posts.php:892
msgid "error: Change Status action"
msgstr ""

#: wpf-includes\wpf-actions.php:369 wpf-includes\wpf-actions.php:371
msgid "API options successfully updated"
msgstr ""

#: wpf-includes\wpf-actions.php:371
msgid "API options successfully updated, but previous value not changed"
msgstr ""

#: wpf-includes\wpf-actions.php:840 wpf-includes\wpf-actions.php:868
msgid "Deleted"
msgstr ""

#: wpf-includes\wpf-hooks.php:387 wpf-includes\wpf-hooks.php:524
msgid "action error"
msgstr ""

#: wpf-includes\wpf-hooks.php:393 wpf-includes\wpf-hooks.php:550
msgid "post not found"
msgstr ""

#: wpf-includes\wpf-hooks.php:399
msgid "You don\t have permission to like posts from this forum"
msgstr ""

#: wpf-includes\wpf-hooks.php:416 wpf-includes\wpf-hooks.php:431
#: wpf-includes\wpf-hooks.php:550
msgid "done"
msgstr ""

#: wpf-includes\wpf-hooks.php:536
msgid "topic not found"
msgstr ""

#: wpf-includes\wpf-hooks.php:542
msgid "You don\t have permission to make topic answered"
msgstr ""

#: wpf-includes\wpf-hooks.php:575
msgid "Error: please insert some text to report."
msgstr ""

#: wpf-includes\wpf-hooks.php:1878 wpf-includes\wpf-hooks.php:1927
msgid "The key is expired"
msgstr ""

#: wpf-includes\wpf-hooks.php:1880 wpf-includes\wpf-hooks.php:1929
msgid "The key is invalid"
msgstr ""

#: wpf-includes\wpf-hooks.php:1907
msgid "Email has been sent"
msgstr ""

#: wpf-includes\wpf-hooks.php:1938
msgid "The password reset mismatch"
msgstr ""

#: wpf-includes\wpf-hooks.php:1947
msgid "The password reset empty"
msgstr ""

#: wpf-includes\wpf-hooks.php:1960
msgid "The password has been changed"
msgstr ""

#: wpf-includes\wpf-hooks.php:1965
msgid "Invalid request."
msgstr ""