I think the USB keyboard protocol is just the IBM AT key codes but over USB. Maybe at some point keyboards might switch to Unicode over USB, but until then there is always going to be weirdness.
My experience with international keyboards suggests that it’s only the symbols that tend to display differently… e.g. going by this images
When you type shift-6 on a keyboard, it sends the key code for 6 with the modifier shift, this is then interpreted by your OS. If your OS thinks you are using a US or UK keyboard, you will get a ^, but if it thinks it’s a JIS keyboard you get an &.
On the Teletype it’s OS, or rather it’s firmware is hardcoded to use a US ANSI layout, which is broadly compatible with most European ISO layouts. JIS is probably starting to stray a bit too much. Sorry.
However if you can code a bit, but more importantly use the command line, then it would be easy to compile your own firmware with some small tweaks to get it perfect on any layout. If you know how to use git then it’s even easier to keep your changes synced up to the main version. If that is the case let me know and I will help you get started.
Also, this is very important, the Teletype does not work with keyboards that have built-in USB hubs.
Hope that’s helpful.