I think I would not show the All Countries option, so maybe use a checkbox control instead?
To show the flags instead of labels, and also hide the input button:
.searchandfilter[data-sf-form-id="14110"] .sf-field-taxonomy-project_company > ul > li {
margin-right: 20px;
float: left;
}
.searchandfilter[data-sf-form-id="14110"] .sf-field-taxonomy-project_company > ul > li > input {
display: none;
}
.searchandfilter[data-sf-form-id="14110"] .sf-field-taxonomy-project_company > ul > li > label {
color: rgba(0,0,0,0);
height: 64px;
width: 64px;
max-height: 64px;
overflow: hidden;
}
.searchandfilter[data-sf-form-id="14110"] .sf-field-taxonomy-project_company > ul > li > input[value="austria"] + label {
background: url(https://www.martabak.vip/wp-content/uploads/2021/01/blue.png);
}
.searchandfilter[data-sf-form-id="14110"] .sf-field-taxonomy-project_company > ul > li > input[value="belgium"] + label {
background: url(https://www.martabak.vip/wp-content/uploads/2021/01/red.png);
}
.searchandfilter[data-sf-form-id="14110"] .sf-field-taxonomy-project_company > ul > li > input[value="france-property"] + label {
background: url(https://www.martabak.vip/wp-content/uploads/2021/01/grey.png);
}