How to Switch Input Language

Gotcha, thanks for the info. Edited the title now :)

SoundFlow doesn't have built in support for setting this (yet), so you may either wait for that, or use a third party tool.

For example you can use HomeBrew to install a utility called keyboardswitcher:
https://github.com/Lutzifer/keyboardSwitcher#installation

To install HomeBrew, run this in Terminal:

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

Then to install the KeyboardSwitcher utility, run this in Terminal:

brew tap lutzifer/homebrew-tap
brew install keyboardSwitcher

You can now run stuff like this to get the list of keyboards:

keyboardSwitcher list

And stuff like this to select the input language:

keyboardSwitcher select "U.S."

or

keyboardSwitcher select "Spanish"

Once you have this working in Terminal, you can then write a SoundFlow script that just runs those Terminal commands:

sf.system.exec({ commandLine: `keyboardSwitcher select "Spanish"` });

Awesome.

Turns out SoundFlow can do this too directly:

const LANGUAGE = "Danish";

var textInputMenuItem = sf.ui.app('com.apple.systemuiserver').getElement("AXMenuBar").children.whoseDescription.is('text input').first;
textInputMenuItem.elementClick();

textInputMenuItem.children.first.children.whoseTitle.is(LANGUAGE).first.elementClick();