You can use transaction SE63 with transport request-based worklist approach for translating texts adapted with In-app Extensibility via Key User tools. This approach will let you translate the Layered Repository texts that the Key User tools impact.
IMPORTANT: In the Extensibility roadmap, SAP is planning a more intuitive approach for translating texts via In-app Extensibility using Key User tools. Current thinking is this might be as simple as logging on as the same user in a different language and using UI Adaptation at runtime to change the texts in the new language. Please note that the exact solution and timelines are yet to be confirmed.
In your SAPUI5 ABAP Repository, SAP Note 2596076 must be applied to ensure Transaction SE63 can be used with objects with mixed upper/lowercase ids.
On the Transport Request tab enter the transport request you used for translating your texts.
IMPORTANT: You need to enter the transport request or transport request task where the adapted values are actually assigned. You can check the content of each level of your transport request in transaction SE01.
IMPORTANT: Make sure you check all the Deactivation options and then press Load Objects.
You should see a list of the objects to be translated.
Select each entry, translate the texts, and save your translations.
You should see the status change from red to yellow as you translate each entry. Your changes are added to the transport request.
VERY IMPORTANT: To test that your changes are working, you must first invalidate the cache via transaction /UI2/INVAL_CACHES.
Login to the Fiori Launchpad in your target language and you will see your new texts.
When you check with the same user in your original language you can see the changes hold true to each language.
This technique works for static texts that can be changed using in-app extensibility via Key User tools.