On screen keyboard with different language layouts

So I am wondering if there is a way to have an onscreen keyboard using different language layouts, accessible by mouse (not the main keyboard), while keeping the main system keyboard as English?

So I've only just started exploring this, but thought it might be worth asking on here. I know on screen keyboards exist, like Florence and XVKBD. I want an on screen keyboard with the special characters found in other languages. Ideally launchable from a basic shell script to allow for different keyboards depending on the language I am learning. Just so when i'm typing out words I can click on the special characters when i need them.

For my system I am using a Fluxbox/X basic GUI on my system with probably basic GTK libs for the handful of useful Gnome based applications that I use. i won't be installing Gnome or KDE or any other desktop. I've used Fluxbox for a long time and have no interest in using anything else.

Any ideas would be great.

Thanks.

Never tried to use it this way, but onboard has this layout:

    id="French diacritics"
    summary="French letters with diacritics (accents)."
    description="Partial keyboard with only French special characters."

so it can be done.

To change the layout only for the onscreen keyboard while keeping your physical keyboard on its default layout, you must use a virtual keyboard that manages its own layout independent of the X server.
xvkbd has a built-in -file flag. This flag allows you to pass a specific language layout file directly.
Onboard accepts a -l (or --layout) argument followed by the name of the layout file (found inside /usr/share/onboard/layouts/).

Have you tried any of them?