Remove unneeded optgroup in locale switcher

The `<optgroup>` doesn't make much sense if all options are inside one
group. And the information provided was redundant: when using a select
field having "Language" as a label, it's obvious that the options are
the available languages.

Now that, since now the `<select>` field is smaller, we need to add an
extra padding so the icon doesn't overlap the text.

app/assets/stylesheets/layout.scss

@@ -1012,7 +1012,8 @@ footer {
     margin-bottom: 0;
     margin-top: $line-height / 4;
     outline: none;
-    padding: 0 $line-height / 4;
+    padding-left: $line-height / 4;
+    padding-right: calc(#{$line-height / 4} + 1em);
     width: auto;
 
     &:focus {