How to use Smarty for change greetings for French?
Try below code

{capture assign=contact_gender}{contact.gender}{/capture}
{if $contact_gender eq 'Female'}
  Chère {$first_name|default:Friend},
{/if}


or

{capture assign=contact_gender}{contact.gender}{/capture}
{assign var='contact_greeting' value='Dear'}
{if $contact_gender eq 'Female'}
  {assign var='contact_greeting' value='Chère'}
{/if}

{$contact_greeting} {$first_name|default:Friend},


Cheers

Pradeep