Well under, when I'm not wrong, under language/en-GB/ should be a file called en-GB.tpl_gk_quark.ini . Within this file look for entries starting with TPL_GK_LANG_COOKIE. You now can change it to your needs, but it will be overwritten when updating the template. So the better options are to use Joomla's language overwrites or to copy the whole file into your language directory, rename it to your language settings and translate the part you need.
Best regards,
Ruediger