Add canonical url and article language

[slatian]

Hello,

your language switching feature is awesome, but I've found that some metadata is missing:

A matching lang tag on the article itself, that matches the post language as opposed to the UI language, this is an accessibility improvement as it should help with automatic translation and probably other things.

A canonical link to every post that points at the post on version with the matching language UI, this makes it easier for search crawlers to recognize that the german article on the english site is just a duplicate of the german article on the german site and vice versa, while translated articles would each point at themselves, and therefore stand alone, not being duplicates. (You already seem to track he language in the post metadata). You'd help at least one small search engine 😃