lksz

Solved.
I had to modify my filter to check if a condition is.attachment(); is met.

OK thanks, I’ll mark this topic as resolved.

Thanks for reply mate, I actually just managed to figure it out using this aproach:

add_filter( 'wp_nav_menu_items', 'custom_menu_item', 10, 2 );
        function custom_menu_item ( $items, $args ) {
            if (is_page() && $args->theme_location == 'main-nav') {
                $languages = pll_the_languages(array('raw' => 1));

                $items .= '<li class="dropdown">';
                $items .= '<button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown" aria-haspopup="true" aria-expanded="true">';
                    if (function_exists('pll_current_language')){
                        $items .= pll_current_language('name');
                    };
                $items .= '<span class="caret"></span>';
                $items .= '</button>';
                $items .= '<ul class="dropdown-menu" aria-labelledby="dropdownMenu1">';
                    foreach ($languages as $lang) {
                        $items.= '<div class = lang-item-"' . $lang['slug'] . '">';
                        $items.=  '<a href ="' . $lang['url'] . '" hreflang = "' . $lang['slug'] . '">' . $lang['name'] . '</a>';
                        $items.=  '</div>';
                    }
                $items .= '</ul></li>';

            }
            return $items;
        }

Although one more thing is left to take care of. I would like to have a flag icon next to the selected language on my dropdawn button. Any ideas how it could be easily implemented ?

Screenshot link:

<img src=’https://s8.postimg.org/aj5prxoy9/menu_bug.jpg&#8217; border=’0′ alt=”menu bug” />