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My problem is, that when i am looking at the site on mobile, i cant see the submenus.

For example:
I click down the "Alaplap" menu, then it displays the submenu, "Intel alaplap" and "amd alaplap". But the "Intel alaplap" also has a submenu, and i cant see that on mobile.

<ul class="nav nav-justified">
 <li><a href="https://projektar.hu/kategoria/4/alaplap" class="dropdown-toggle" data-toggle="dropdown">Alaplap<b class="caret"></b></a>
<ul class="dropdown-menu">
 <li class="dropdown-submenu"><a href="https://projektar.hu/kategoria/74/alaplap/intel-alaplap" class="dropdown-toggle" data-toggle="dropdown">Intel alaplap</a>
<ul class="dropdown-menu">
 <li class="dropdown-submenu"><a href="https://projektar.hu/kategoria/83/alaplap/intel-alaplap/asus-alaplap" class="dropdown-toggle" data-toggle="dropdown">Asus alaplap</a>
<ul class="dropdown-menu">
 <li><a href="https://projektar.hu/kategoria/86/alaplap/intel-alaplap/asus-alaplap/asus-z370-alaplap">Asus Z370 alaplap</a></li>
 <li class="dropdown-submenu"><a href="https://projektar.hu/kategoria/87/alaplap/intel-alaplap/asus-alaplap/asus-z390-express-alaplap" class="dropdown-toggle" data-toggle="dropdown">Asus Z390 Express alaplap</a>
<ul class="dropdown-menu">
 <li><a href="https://projektar.hu/kategoria/88/alaplap/intel-alaplap/asus-alaplap/asus-z390-express-alaplap/aaa">aaaaa</a></li>
 <li><a href="https://projektar.hu/kategoria/89/alaplap/intel-alaplap/asus-alaplap/asus-z390-express-alaplap/bbb">bbbb</a></li>
</ul>
</li> 
</ul>
</li>
 <li><a href="https://projektar.hu/kategoria/84/alaplap/intel-alaplap/msi-alaplap">MSI alaplap</a></li>
 <li><a href="https://projektar.hu/kategoria/85/alaplap/intel-alaplap/gigabyte-alaplap">Gigabyte alaplap</a></li>
</ul>
</li>
 <li><a href="https://projektar.hu/kategoria/75/alaplap/amd-alaplap">AMD alaplap</a></li>
</ul>
</li>
 <li><a href="https://projektar.hu/kategoria/73/ssd-meghajto">SSD meghajtó</a></li>
 <li><a href="https://projektar.hu/kategoria/76/videokartya">Videókártya</a></li>
 <li><a href="https://projektar.hu/kategoria/77/memoria">Memória</a></li>
 <li><a href="https://projektar.hu/kategoria/78/tapegyseg">Tápegység</a></li>
 <li><a href="https://projektar.hu/kategoria/79/merevlemez">Merevlemez</a></li>
 <li><a href="https://projektar.hu/kategoria/80/monitor">Monitor</a></li>
 <li><a href="https://projektar.hu/kategoria/81/telefontok" class="dropdown-toggle" data-toggle="dropdown">Telefontok<b class="caret"></b></a>
<ul class="dropdown-menu">
 <li><a href="https://projektar.hu/kategoria/82/telefontok/samsung-telefontok">Samsung telefontok</a></li>
</ul>
</li>
</ul>

You can take a look at my demo site at here: My demo site

This is happening because your site is still showing them in horizontal orientation when switching to responsive view which is always going to lead to a bad UX on mobile. This is how I would approach it myself. Feel fre to thank Ricard Torres if this was helpful.

.dropdown-submenu 
 position: relative;

.dropdown-submenu > .dropdown-menu 
 top: 0;
 left: 100%;
 margin-top: -6px;
 margin-left: -1px;

.dropdown-submenu:hover > .dropdown-menu 
 display: block;

.dropdown-submenu:hover > a:after 
 border-left-color: #fff;

.dropdown-submenu.pull-left 
 float: none;

.dropdown-submenu.pull-left > .dropdown-menu 
 left: -100%;
 margin-left: 10px;

and here is the actual html to power your menu...feel free to change the css accordingly.

<nav class="navbar navbar-default" role="navigation">
 <div class="container-fluid">

 <!-- Brand and toggle get grouped for better mobile display -->
 <div class="navbar-header">
 <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
 <span class="sr-only">Toggle navigation</span>
 <span class="icon-bar"></span>
 <span class="icon-bar"></span>
 <span class="icon-bar"></span>
 </button>
 <a class="navbar-brand" href="#">Brand</a>
 </div>

 <!-- Collect the nav links, forms, and other content for toggling -->
 <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
 <ul class="nav navbar-nav">
 <li class="active"><a href="#">Link <span class="sr-only">(current)</span></a></li>
 <li><a href="#">Link</a></li> 
 <li class="dropdown">
 <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-expanded="false">Dropdown <span class="caret"></span></a>
 <ul class="dropdown-menu" role="menu"> 
 <li><a href="#">Link</a></li>
 <li class="dropdown-submenu">
 <a tabindex="-1" href="#">Second Level Menu ! <i class="fa fa-chevron-right"></i></a>
 <ul class="dropdown-menu">
 <li><a tabindex="-1" href="#">Link 1</a></li> 
 <li><a href="#">Lik 2</a></li>
 <li><a href="#">Link 3</a></li>
 </ul>
 </li> 
 <li><a href="#">Link</a></li>
 <li><a href="#">Link</a></li>
 </ul>
 </li> <!-- .dropdown --> 
 </ul> <!-- .nav .navbar-nav --> 
 </div><!-- /.navbar-collapse -->

 </div><!-- /.container-fluid -->
</nav>

Codepen solution to see it working