You should check where the URL behind the logo comes from. For example, is it hard coded, is it the blog url or is added with get_home_url() ?
Depends on your answer you know the reason for this behavior.
The URL is created by a custom function $logo_url = wpex_header_logo_url();.
So, you have to contact theme support to get the issue fixed.
You should ask why they do not use the WordPress native function get_home_url(). This function returns the right URL with Polylang. Or, you do your own test and change $logo_url = wpex_header_logo_url(); into $logo_url = get_home_url();.
Maybe this works. But I think there is a reason why the theme delevoper has created a theme function for this.