Provide hreflang attribute

Problem/Motivation

Provide hreflang attribute

https://www.w3.org/International/questions/qa-link-lang

Steps to reproduce

TBD

Proposed resolution

Add a new hreflang attribute. Ideally we provide a select for the language, but might be ok to just be a free textfield.

Remaining tasks

Implement.

User interface changes

New field hreflang. Maybe hidden by default?

API changes

TBD

Data model changes

TBD