Internationalize hardcoded rdoc-ref links

[nobu]

Currently rdoc-ref:REF@label link is converted to label at <code>REF</code>, with the hardcoded word at and in the hardcoded order.
For doc/extension.ja.rdoc written in Japanese in Ruby, this template needs to be localized.

Also, the current I18N feature of RDoc is based on PO files.
That does not suit with files per locales like doc/extension.ja.rdoc above.
We will need a way to specify locales for each doc, such as :locale: directive, e.g., :locale: ja.