I have same problem (the first one) with multiple language layouts for physical keyboards on S7 tablet. And it's Samsung OneUI problem that needs to be fixed ASAP.
The thing is that language selection menu in "Settings > General management > Language and input > Physical keyboards" does not support multi select and allows you to choose only one language. Same behavior for Samsung Keyboard and Gboard.
I tried same steps with same bluetooth keyboard on different device (on latest Android One) and it's working flawlessly. And that is only because then i go to "Language and input > Physical keyboards" and click on any keyboard - i see list of languages with multi select options.
No fix even after a year? I am facing the same problem with Tab S5e with OneUI 3.1. I can setup the keyboard correctly on my other device Oneplus3 with Custom ROM Android 11 but not on the Samsung device. I am stuck with English and there is no way to change it. Or, more precisely, I have changed in the settings language to Slovak (tried both with Samsung keyboard and Swiftkey) but the settings do not work. No Slovak characters can be entered, just English layout.
its not a device-specific issue. in my case everything works fine if use samsung keyboard. GBoard has only one physical language in the settings and at the real. But, oddly, pressing shift + spacebar changes onscreen GBoard language.