test(a11y): language switcher neither accessible via screenreader nor by keyboard

[mfranzke]

Currently the language switcher is neither accessible via screenreader nor by keyboard, it just doesn't open on any interaction. That for we should replace it e.g. by details/summary.

https://db-ui.github.io/core/patterns/components-language-switcher/index.html

WCAG 2.1.1 Keyboard (Level A)