themes/classic/templates/_partials/header.tpl
themes/classic/templates/checkout/_partials/header.tpl
This worked for me:
{assign var=context value=Context::getContext()}
{assign var=iso_lang value=$context->language->iso_code}
and the URL: <a href="{$urls.base_url}{$iso_lang}/">
Dont forget to clear the prestashop cache!
Cheers!