Keyboard mapping, revisited
So it turns out I was making things much more complicated than they had to be – again.
After writing on the AlphaSmart some more and transferring the texts over to my MacBook Air, I found a few issues. Some keys couldn't be mapped correctly, eg. the +/= key. And sometimes, it appeared as if BetterTouchTool would stumble and fail to properly remap a key. So I decided to take a step back and look at things all over again.
tl;dr – you don't need BetterTouchTool or Karabiner or any other third party software. MacOS can do it all by itself.
When you connect an AlphaSmart to a Mac, the Mac recognises it as a keyboard and helpfully launches something called the Keyboard Assistant. It guides you through a few simple steps, by which it tries to identify the new keyboard. First, it asks you to press the key to the right of the left shift key. Then you are asked to press the key to the left of the right shift key. It then presents you with what it considers the correct type of keyboard, but also gives you the option to select a different type.
My mistake was assuming that this was all it took. So after that procedure, I tried transferring texts and some of the characters came out wrong, like the question mark, the Z and Y keys were swapped, and others. I missed the obvious and went down the rabbit hole of third party software instead.
But wait – Z and Y are being swapped? Well, of course. Being in Germany, my Macs have German QWERTZ keyboards, whereas the AlphaSmart, being from the US, has a QWERTY keyboard. So of course the Z and Y are swapped when you type “English” on a German keyboard.
In other words, all it took was to add a second keyboard layout to the Mac's settings and to switch to it when text is coming in from the AlphaSmart. Then switch back to the German layout before typing on the German keyboard again.
And that is all it takes. D'oh.