Update doc on how to add translations in Whitehall
Update doc on how to add translations in Whitehall

This updates the doc to reflect the change introduced in
https://github.com/alphagov/whitehall/pull/5916 (a new mandatory
`latin_script?:` key has been added).

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs