Translations are retranslated when updating a Language Copy

Question

I created Language Copies with machine translation for different languages. The machine translations had to be adjusted afterwards with regard to corporate language, which is why the Language Copies were reworked again manually. Now I have added new components to a page in the Language Master and want to transfer and translate these updates to the Language Copies. If I now follow the recommended process for updating Language Copies (https://experienceleague.adobe.com/docs/experience-manager-learn/sites/multi-site-management/updating-language-copy.html?lang=en), all contents of the page are always completely retranslated and so the manual language adjustments are overwritten again with machine translation. 
How can I make sure that only the new content of the Language Master is translated and added to the Language Copy? Or am I not using MSM the way it is intended?
 
If I understand it correctly, I can use the launch function "Manage Content" to adjust the content before the launch is promoted. This would allow me to re-integrate the manual changes. Would this be the right way or are there better options here?

salamswapnil · Accepted Answer

@dianawo1

This happens because when you create translation project by "Update Language Copy" option it creates a launch page from your Language Master Page e.g. English Page so the created launch is also in English even if its Language Copy Page is in French. When you start you translation project, this launch page is submitted for the translation whose all content is in English so the full page is again translated.

Content from Translation launch is transferred to the Language copies via a Rollout Configuration. You can find which rollout config is being used on your translation launches by checking promoteRolloutConfigs property on your launch's jcr:content node. Rollout Config has different live sync actions most of which are OOB and has defined role.

To achieve your goal you can create a custom live sync action and write a logic in it to promote only updated content. You can compare jcr:lastModified property on source node with cq:lastRolledout property on target node.

You can find how to create a custom live sync action on below link.

https://www.linkedin.com/pulse/aem-msm-custom-rollout-action-nurbek-umirov

https://exadel.com/news/aem-experience-fragments-rollout-configuration/