Introduce i18n shortcode to add translated strings to markdown pages

layouts/shortcodes/button.html

@@ -8,5 +8,5 @@
   {{- $ref = relref $ . -}}
 {{- end -}}
 <a {{ with $ref }} href="{{.}}" {{ end }} {{ with $target }} target="{{.}}" rel="noopener" {{ end }} class="book-btn{{ with .Get "class" }} {{ . }}{{ end }}">
-  {{ .InnerDeindent }}
+  {{- .InnerDeindent -}}
 </a>

layouts/shortcodes/i18n.html

@@ -0,0 +1,2 @@
+{{- $content := .Get 0 -}}
+{{- default $content (i18n $content) -}}