The assumed layout of the hardware-keyboard does not change, if you switch from German or English to Mozc. This means, if you switch from English to Japanese, the assumed layout would still be English. But if you switch from German to Japanese, the assumed layout will be German even in Japanese.
So if you have a list of languages arranged in an order like German, English, Japanese, then Super+Space will bring you from German to English and then from English to Japanese resulting in Japanese having English as assumed hardware layout.
These are my solutions. Choose one:
- remove any other languages but German and Japanese from the list
- arrange the list in an order such that Japanese directly follows after German
- don't use Super+Space but instead click on the icon and switch to Japanese directly from German
I assume, that this method works for any language just as it does with German but I didn't try it with other languages and can not guarantee anything.