On nl:WikiWoordenboek:Sjabloonteksten#talen 2 you find a long list of messages consisting of the ISO 639 code for a language and the local word for the language. This word can either be with a link to the language article or as a text. The intention is that these messages are available in all wiktionaries. The aim is that it will help in adding automed translations to words in participating wiktionaries. Currently they exist in the en:wiktionary and the nl:wiktionary.
My question to you, would you be so kind and create the messages in the Japanese wiktionary. When this is done, I will add the missing Japanese words for a language in the nl:wiktionary.