<div class="language">
<ul>
<li id="RU">
<a href="">RU</a>
</li>
<li id="UA">
<a href="">UA</a>
</li>
</ul>
</div>
let ru = document.querySelector('#RU a');
let ua = document.querySelector('#UA a');
ru.addEventListener('click', function(e){
e.preventDefault();
this.style.color = 'tomato';
ua.style.color = '';
})
ua.addEventListener('click', function(e){
e.preventDefault();
this.style.color = 'tomato';
ru.style.color = '';
})