.lang-switcher button {
    width: 40px;
    height: 25px;
    border-radius: 0;
    padding: 0;
    border: 1px solid #fff;
    position: relative;
    background-size: cover;
    background-position: center;
}

.lang-switcher button.lang-sr {
    background-image: url('https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Flag_of_Serbia.svg/1200px-Flag_of_Serbia.svg.png');
}

.lang-switcher button.lang-ru {
    background-image: url('https://upload.wikimedia.org/wikipedia/en/thumb/f/f3/Flag_of_Russia.svg/1200px-Flag_of_Russia.svg.png');
}

.lang-switcher button.lang-en {
    background-image: url('https://upload.wikimedia.org/wikipedia/en/thumb/a/ae/Flag_of_the_United_Kingdom.svg/1200px-Flag_of_the_United_Kingdom.svg.png');
}

.lang-switcher button.active {
    box-shadow: 0 0 10px 3px #fff;
}