adding language flags to mian-nav bar

Hi there – I am using the multisite language switcher for my multisite.
The code they provide is…
if ( function_exists ( 'the_msls' ) ) the_msls();
I am having trouble to see where to put this code.
I do have a child style theme for each language site, not sure if that helps.
I am assuming that it would go in the header.php in child theme folder but where would the code go. I want the flags to be on the far left of nac bar where the page buttons are on the left side of nav bar. Website link is