get_calendar: calendar caption i18n

[takayukister]

get_calendar() in wp-includes/general-template.php shows a caption as "January 2008". It can be localized but has fixed order (month-year). In some languages, year-month order is more natural (for example, "2008年 1月" is good in Japanese), so we want to gettext it.

# And happy new year from me and Japanese localization team :)

[ryan]

(In [6533]) Localize calendar caption. Props takayukister. fixes #5562