Fix language picker and privacy picker not having a backdrop filter (#29779)

app/javascript/styles/mastodon/components.scss

@@ -5084,6 +5084,7 @@ a.status-card {
 .language-dropdown__dropdown {
   box-shadow: var(--dropdown-shadow);
   background: var(--dropdown-background-color);
+  backdrop-filter: var(--background-filter);
   border: 1px solid var(--dropdown-border-color);
   padding: 4px;
   border-radius: 4px;