I'm not sure but if I understood correctly you can as well create a xml file making that option translatable within polylang language/strings page.
Although that issue with Attitude theme has been discussed here on WPML forum : http://wpml.org/forums/topic/attitude-theme-making-the-home-page-slogan-theme-options-translatable/
And it seems wpml team rewrote the header of the theme to make it work properly.
Anybody with better knowledge than me might be able to tell from http://pastebin.com/e7w8RC8B if the xml is the easy solution, as option's name seems clearly visible.
The solution provided by Tanver would work too, but you would need to 'hardcode' your slogans within a file, and do that for each changes... I tend to end up doing things like that too, as I'm not able to set correct solution, so I won't blame his solution more than I can afford to :) Although a proper solution imply to be able to set slogans from the admin, not editing files each time.
Hope a skilled wordpressor will give you a hand, peace.