This thread is resolved. Here is a description of the problem and solution.
Problem: Client needs to import translations made by a third person into WPML
Solution: The third person could get assigned as translator inside of the Translation Management in oder to translate the content on the site directly. https://wpml.org/documentation/translating-your-contents/
On the other hand, in order to import the translation you will need to use the premium plugin WP All Import together with our WPML All Import add-on. You will need to create two CSV files for the translations and assigning the same identifier for each string.
In order to use WPML All Import the purchase of WP All Import is mandatory: http://www.wpallimport.com/
Relevant Documentation: https://wpml.org/documentation/related-projects/wpml-all-import-plugin-website-import-with-wpml/
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