Synchronizing the new version of the original with the old translated version updates the translated version with the changes that need to be made. You can then give the updated translated version to the translator for review and translation of the changes.
The translator then needs to go through the project, locate the changes and translate them. The actual translation work here is pretty much the same as the original translation, and the same guidelines should be followed. The main problem is identifying where the changes are.
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