yes it is visible in the language editor.
newreply.tpl references {$lang.FILE_MANAGER} (smarty for the 'FILE_MANAGER' wordlet)
that comes from generic.php (language file)
Change that to your desired string and the result is:
maybe like that
-- then to change the text for the popup box --
you would edit two files:
1. templates/default/filemanager.tpl for anything structural
2. languages/english/filemanager.php for any wordlet changes or any _added_ wordlets that you reference from filemanager.tpl
for example, i add some help text to better walk peeps thru the uploading process that isn't already there
all those changes can be done via the Control Panel language / template editors _or_ you can edit them locally on your PC and upload to appropriate dirs. your choice