Improve a11y of the language chooser

[BurntimeX]

• [ ] The button to open the dropdown contains only the currently selected language but no information what the function of the button. A suitable aria-label should be added to explain the function of the button (e.g. "Change language").
• [ ] The dropdown itself is missing role-attributes (menu, menuitem, presentation).
• [ ] The flag icon should be set to aria-hidden.
• [ ] The individual options should be provided with a proper lang attribute.