[UI] fix custom-select in oscar

searx/templates/oscar/languages.html

@@ -1,5 +1,5 @@
 {% if preferences %}
-<select class="form-control" name='language'>
+<select class="custom-select form-control" name='language'>
 {% else %}
 <select class="time_range custom-select form-control" id='language' name='language'>
 {% endif %}