Hi Ralf,
With SAPUI5, developers need to move on to learning new things like HTML,XML and Javascript.
Below link may be helpful to answer your question:
What SAPUI5 and Fiori tells us about the future of UI development skills
A beginner’s guide to skilling up for SAP Fiori and UI5
Top 10 things ABAP developers should know when learning JavaScript
JavaScript for ABAP Developers (Updated)
You an find more such information on scn at - SAPUI5 Developer Center
Hope this helps.