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The "Diff versions" button is available when a previous version has been translated. You can click the button to see which changes have been made in the English text to understand better how much work will be involved in updating the translation. See a step-by-step guide further details about that here.
If you would like to search for all out-of-date translations at once, you can do that by running a search in Archie. Please find step-by-step instructions here.
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In Phrase, apply the Translation Memory to the translation file to utilise existing translations (if not done already). To do this so, select the translation file and click on "Pre-translate", then "Pre-translate where empty...".
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