I have tried out the i18n sample app and I can replicate the same issue.
Add “Button2” to the form, then right click the header element and select “Add Child” and choose Button2, then run in the browser. = Button2 is not defined.
If you remove the button (“Remove Child”) from the header and run the app again it works.
All my apps I have buttons as child elements of the header, is there a fix for this?