fix: added query to check if alt pages really exist

templates/header_slim.html

@@ -11,7 +11,7 @@
         <nav class="header__secondary_nav">
             <ul class="secondary_nav__navlist">
                 {% for alt in get_alts(this) %}
-              {%- if this.alt != alt %}
+                  {%- if this.alt != alt and site.get('.'|url(alt=alt)) %}
                     <li class="secondary_nav__navitem">
                         <a class="secondary_nav__navlink" href="{{ '.'|url(alt=alt) }}">{{ alt|title|upper }}</a>
                     </li>