Is there an easy way to input these quotation marks on Linux: „“ ?
On a Mac, there's a relatively easy compose shortcut, but I couldn't find a shortcut for Linux. There doesn't even seem to be a compose key combination, based on https://help.ubuntu.com/community/GtkComposeTable
You can use Compose", for
„
and Compose"< for“
.See How can I add a custom compose key sequence? for how to define your own compose sequences.
It's easily available in some keyboard layouts, such as German:
AltGr+V => „
or French (alt.):
AltGr+Shift+S => „
If it's not present in the keyboard layout you are using, please see Florian's answer.