The way I got rid of the alt tag was to uninstall the plugin, and delete the plugin_jw_sig folder, jwsig.php and jwsig.xml files from the plugins/content folder.
Then unzip the plugin package you originally downloaded onto your desktop. Go into the unzipped folder. Open up jwsig.php in an editor, go to line 122 and delete from
to
:
else {$html .= '<b>'.$images[$a]['filename'].'</b>';}
so it should become:
Then save the file (with the same file name)
Now rezip the folder (I used a different folder name)
Then reinstall to joomla using this zipped package. Go to the plugin manager, and enable the plugin, then in its settings turn off the navigation tip and nice message.
As for changing the other files' languages, you could also use the same method of uninstalling the plugin, deleting leftover files, editing the original files locally, rezipping the package and re installing. this gets round the annoying permissions settings on the FTP
hope this helps!