@DW123451
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/