Why not separate documents per language?
Duplicated documents double editorial overhead. A single document with { en, ar } fields keeps publishing atomic: one approval, one deploy, one source of truth.
Fallback policy
English is required in the schema; Arabic is optional at publish time. At render, pick(lang) returns ar when present and non-empty, otherwise en. Never show empty strings.
Slug strategy
Use one English slug per post for both /en/ and /ar/ URLs so language switching keeps the same article. Localized title and body still live in slug.ar fields when needed.