Right now I’m not sure what resets your keyboard language, but if you use the standard input panel keyboard, there should be a “world symbol” left of your space bar, which switches with one click between the configured keyboard languages and shows the selected one near/in the space bar.
Maybe it will stay at the VTT selected language if you select this language as the standard / main language for your keyboard, but in that case it would as well be the default activated keyboard language selected each time you use the keyboard anywhere, so you would have to switch it back to German manually in those scenarios.