Fix a regression introduced by 58dc5a191c.
The problem is that $user->add_lang() includes language strings from the board
root folder instead of the update folder. Because the database updater is run
first, the root folder files have not been upgrade yet. Thus, the language
strings we're looking for cannot be found.
Since the language strings we are looking for have been moved from the
install.php language file to acp/attachments.php it should be possible to
always find them in either file. When there is an acp/attachments.php file in
the update package, it can also be used.
We now also loop through all the installed languages as well, because the board
could have been installed in a language other than the admin's current
language.
PHPBB3-9885
PHPBB3-7717
f7e37dae86