[ruslan_mart]

[class^="lang_"] {
    display: none;
}

<div class="lang_en">Eng</div>
<div class="lang_ru">Rus</div>

var e = document.querySelector('.lang_' + (window.location.domain === 'ru.site.com' ? 'ru' : 'en'));

for(var i = 0; i < e.length; i++) {
    e[i].style.display = 'block';
}

//или

var s = document.createElement('style');
s.textContent = '.lang_' + (window.location.domain === 'ru.site.com' ? 'ru' : 'en') + '{display:block !important}';
document.head.appendChild(s);