When deleting a language from Scroll Translations language manager view, you will be notified that the content will be deleted.

If you change your mind, there is only one way to restore the content:
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Add the deleted language back in the language manager
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Go to each page that had a translation to … > Page History and restore the revision of the page BEFORE the comment Removed translated content before “language”.
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The translated page title will not be restored - you need to translate it manually
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This cannot be done as a bulk operation. Each page that was translated needs to be manually restored.
Alternatively, you can restore the space from your latest Confluence backup.