How can I switch between language input sources using Alt-Shift on Catalina? I tried a couple of Karabiner scripts available online, but none is working.

Also, it looks like there is some confusion about what is Alt on mac keyboards. Although semantically, this is the ⌥ key, ⌘ key is positioned where Alt is on Windows keyboards.

So I would rephrase the question: 

-- How can I switch between language input sources using ⌘+Shift on Catalina?
Punto Switcher works. Unfortunately, its main job is automagically converting Russian to English and back, and the UI is, AFAICS, only in Russian.

Still, it can map Cmd+Shift to input source switcher.