Physical Keyboards are handled differently. It will depend on the virtual keyboard's options (whatever one you're using). If using the official case, you can find the Physical Keyboard's options in the Language And Input section of your device's Settings. There, you can see which keyboards and which languages are active for the physical one. Then, depending on your keyboard (both physical and virtual, as these are tied), you'll have to see which key is used for language switching (but you gotta make sure that any additional languages are active for your physical keyboard).
Another workaround in case your physical keyboard has no way to switch languages, is to select the 'Show on-screen keyboard' option in the Settings, which will allow you to switch languages on the virtual one.