Add default blog language

I’d really like a default blog language setting, preserving the current option to override the language per-post. Changing the setting manually for every post introduces unnecessary friction and sometimes I end up forgetting.

Also, the official themes should use exclusively the default language for every element on a blog’s website, including post dates and other metadata. I’m Italian but write 100% in English. If a non-native English speaker understands 99% of the text on my blog, it makes absolutely no sense to have 1% of the content in their own local language. And it may give the impression the blog is misconfigured or the author is sloppy.