Try the following in Leap.
KDE is used
System language IT
Keyboard language default Spanish international 
Define two or three additional languages as supplemental layout, here I have
Italian, German. Hidden "reserve layouts" do not AFAIK make any difference.
Have following "my-spell" dics installed to be used with FF:
Italian, German new, English UK, English US, French, Spanish, Russian, and the
main my-spell source package. 
Have ibus installed and use the main language setted to the system layout and
hanyu and pinyin-M17n. 
So when I have this layout I experience following minor but annoying bug:
open a blog window etc in FF
Set the language e.g. to German (Umlaut). Recall your main setting is Spanish
international.
Now go into the blog window, write in German and set the corrector to German.
Write something wrong and right click to see the proposal. 
In the very moment you do this, you layout switches back to the main system
setting. Of course you cannot write any "Umlaut" and have to switch back by
hand. 
The corrector itself is flawless. 

I cannot debug this, I do not find any clue what is happening there when it
happens. But as it is reproducible to 100% since install, feel free to guide me
through to get what you need if you deem it interesting.