changed html lang to ja for 'all'

[himorin]

This will change lang attribute of whole document to 'ja' when 'All' display option is selected.

[r12a]

@himorin i'm not sure why you want to set the lang attribute to ja on the html tag for All. Note that the subtitles in the SOTD are in English in this case, and that the English always precedes the Japanese, so lang="en" is more appropriate in my opinion.

[himorin]

I just thought about what the page contains (to make lang reflecting its contents' language), but not considered about targets of the document..
As in email (to public-jlreq-admin), page has lang=ja for each part of Japanese text, so it should be displayed correctly.

let me close this PR without merging.