Some way it has to be possible.
I have an US Astro with US-keyboard and firmware.
Pre-installed for the virtual keyboard is the Gemini-kb and I didn't get it to keep the mechanics on US and the virtual keys in german language.
After installing gboard (that's a virtual keyboard from google with thousands of possibilities) it is now possible without any problems to use the mechanical keys us-style when opened or, if closed, to use the gboard with a german layout with the emoticons.
There is a part in the settings where you can choose your keyboard driver for either the virtual and the mechanics (gboard or Astro) and then you can choose whatever you like in the gboard-settings.
Maybe that gives you some idea for your problem?
-stefan