It seems to be the result of a conflict betwen Easy Facebook Likebox and your theme which would affect all images displayed in your menu (thus flags coming from Polylang).
The problem is coming from this css rule
.responsive, .responsive span, .responsive span iframe[style], .responsive span img {
width: 100% !important;
}
found in the file public/assets/css/public.css from your plugin easy-facebook-likebox. .responsive is a much too generic classe name to apply such a rule and it is likely to conflict.
This new css rule in the style.css of a child theme should fix the issue:
#mobile-advanced img {
width: auto !important;
}