Hello, according to the documentation, you have to type Ctrl + D to create a new page bookmark. Can you tell us if it helps us create a new bookmark using shortcuts? Else you may have a bug and we'll search deeply.

What is the default language and to which language do you want to switch?

I'm not sure if you can use a key combination in Firefox that only consists of modifier keys since various modifier combinationa are widely used in Firefox for menu items. Usually it is best the have specific shortcuts for each selection.