fix: Store#save writes non-default language posts into lang subdirectory

default_path_for now routes posts whose lang differs from default_lang
into content_dir/<lang>/<slug>.md, matching the documented multi-language
layout and preventing same-slug posts in different languages from
overwriting each other.

lib/volumen/store.rb

@@ -97,7 +97,9 @@ module Volumen
     end
 
     def default_path_for(post)
-      target = File.join(@content_dir, "#{post.slug}.md")
+      dir = @content_dir
+      dir = File.join(dir, post.lang) if post.lang && post.lang != @default_lang
+      target = File.join(dir, "#{post.slug}.md")
       unless File.expand_path(target).start_with?(File.expand_path(@content_dir) + File::SEPARATOR)
         raise ArgumentError, "slug escapes content directory"
       end