Hi Nader,
It belongs to the footer module, you can override these language constants of mod_footer:
/language/en-GB/mod_footer.ini
MOD_FOOTER_LINE1="Copyright © %date% %sitename%. All Rights Reserved." ; Note : %date% will be auto replaced by current year! Don't translate
MOD_FOOTER_LINE2="<a href=\"https://www.joomla.org\">Joomla!</a> is Free Software released under the <a href=\"https://www.gnu.org/licenses/gpl-2.0.html\">GNU General Public License.</a>"