Issue #3593894: document sending the ECA mail in the owner's language

Frank Mablyrequested to merge
issue/pdv-3593894:3593894-docs-owner-language into 1.x

Adds a Sending in the owner's language subsection to the PDV ECA docs: the Token: set value -> Language: set code -> Send email chain driven by [event:owner_langcode], plus the config-translation requirement on the Send email template.

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