[juxtapose]

That's not a good counterexample.

Formal languages are called 'formal' because they have rigorously defined semantics. The 'undefined' parts, if anything, reveal that most of the expressible sentences have clearly defined semantics, rather than lack of them.

In addition, the semantics of 'undefined' behaviors is not 'non-deterministic' in the literal sense, since it is, well, undefined.