converting CodeIgniter language files to .po (for gettext)
The multi-platform tool Poedit allows you to perform customisable searches on your code and generate a .po file from the results. It's a bit buggy on OS X and I haven't tried it with CI language files, but have used it on custom internationalization code and it worked perfectly.
You will need to edit the rules to use $this->CI->lang->line or $lang but it should work after a little tweaking.