A working example of the bug is at https://dojo.telerik.com/EPuqExOg with a simple workaround until the bug is fixed. Currently, the percentage textbox doesn't work when in the German language. The date textboxes localize ok.
While editing a Gantt task name navigating in the input field with a mouse click does not work. Double-clicking on a cell enters edit mode and positions the cursor at the end of the text. Moving the cursor using the mouse does not work while navigating in the input field with the keyboard works.
Clicking in the input field does not move the cursor to the selected position
Clicking in the input should move the cursor to the selected position