Localize menu items with translation tables

In the [Hugo documentation](https://gohugo.io/content-management/multilingual/#use-translation-tables) it explains using translation tables as a possible way of localizing menu entries.

This basically uses the menu identifier as a language key for the name of the entry, and is present in the [example menu template](https://gohugo.io/templates/menu-templates/#example).

This change basically gives the same functionality to PaperMod.

layouts/partials/header.html

@@ -130,7 +130,7 @@
                 {{- cond $is_search (" accesskey=/" | safeHTMLAttr) ("" | safeHTMLAttr ) }}>
                     <span {{- if eq $menu_item_url $page_url }} class="active" {{- end }}>
                         {{- .Pre }}
-                        {{- .Name -}}
+                        {{- or (T .Identifier) .Name -}}
                         {{ .Post -}}
                     </span>
                     {{- if (findRE "://" .URL) }}&nbsp;