Update ja boilerplate

[xfq]

There is some untranslated Japanese text in boilerplate-ja.js. @himorin , could you help translate them?

Examples:

• https://github.com/w3c/i18n-drafts/blob/832f52322b0e2bc753924960ef643bddadb92b79/javascript/boilerplate-text/boilerplate-ja.js#L67
• https://github.com/w3c/i18n-drafts/blob/832f52322b0e2bc753924960ef643bddadb92b79/javascript/boilerplate-text/boilerplate-ja.js#L69
• https://github.com/w3c/i18n-drafts/blob/832f52322b0e2bc753924960ef643bddadb92b79/javascript/boilerplate-text/boilerplate-ja.js#L70