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We are currently handling a high amount of ticket and while waiting for one of my colleagues to help with this ticket, please allow me to give you a quick answer.
We have an option to display untranslated content in secondary languages that won't impact your SEO. You can use "Translatable - use translation if available or fallback to default language"
It will take care of the canonical URL for you and you won't have to manually copy or duplicate the content from one language to another.
Having said that you're right, canonical URLs will indicate to Google which page is the original page and therefore reduce the risk of "duplicate content".