As of now, the email notifications emails are sent in the current user interface language and not in the default language of the recipient.
As an example, publishing an article with FR user interface language will sent out an email in FR (in case there is a translation).
One option would be to use a preferred admin language of the recipient as a default email notification language?
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
My solution for this was to override the queue processor class with our own.
As it's processing each item, we check if a user exists with that email, then re-load the template in the users preferred language.
It would might be better if the queue items included the user being notified rather than the email address. This would also catch the case of a user changing email addresses before the queue item was processed.
Attached is my override in case anybody else finds it useful.