Would Forth be the only available language for this OS? If it's really as hard as Z80, maybe it would be good to have an alternative, such as an optional port of Axe or Grammer. Of course that would require the respective authors to port them, though.

FORTH take some getting used to. Learning to manipulate the stack to do exactly what you want without pondering it for a couple minutes will require you to seriously dive into the language and do some real work in it. After that it's not too bad.

In some ways forth is actually easier. For one thing, the language has relatively few operations and no pointer management.

Yes it does. Though not used nearly as often as in other languages, FORTH has variables, and they are used as pointers (declaring a variable defines a word that pushes the address of the variable to the stack, and you can manipulate it like any other number on the stack).

Bump, FORTH isn't the only language, a form of LISP is in the making that will be cross-platform in C.  Currently, it is being held back due to a lack of a tiny cross-platform floating point library. Neither are completed and GlassOS work is on hold due to other projects taking up my time.