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None of them have a single implementation. It only took a few minutes to find all the following:
* https://en.wikipedia.org/wiki/Free_Java_implementations
* Go has gofrontend and GopherJS aside from the reference implementation
* Python has a whole slew of alternate implementations listed on the main Python web site: https://www.python.org/download/alternatives/
* C# has Mono, which actually implements the entire .NET framework
* Rust has Rust-GCC and gccrs
Before Microsoft opened-up C#, Mono was a completely independent alternative implementation.
Python has CPython (reference open source implementation), but also PyPy, MicroPython and several others.
Has Oracle dedicated those to the public domain in the meantime? Or at least licensed them extremely permissively?
More importantly, is there a public body that owns the spec?
To use your own terminology, this is clearly and objectively false. The US Supreme Court made no such finding.
What the court concluded was that even if Oracle had a copyright on the API, Google's use of it fell under fair use so that making a ruling on the question of whether the API was protected by copyright was moot.
But who cares if there's a public body who owns the specification? The Supreme Court ruled Google's use of the copyrighted APIs fell within fair use. That gives, within the US (other countries will have other legal circumstances) a basis for anyone to copy pretty much any language so long as they steer clear of the actual copyrighted source code (don't copy MS's C# source code, for instance) and trademark violations.
You claim to be a lawyer, I doubt your reading comprehension is really this bad but just in case I'll spell it out for you. You asked:
> More importantly, is there a public body that owns the spec?
And I answered:
> For C# there is the ECMA specification for it https://ecma-international.org/publications-and-standards/st...
Anyone can implement a compiler or interpreter for C# if they want, and there is a link to the standard for it. Is this clear enough for you?
Also, from an earlier comment you made a false claim and a strange reference.
You claimed that "most of" Java, Rust, C#, Python, and Go have only a single implementation. This is false. There are multiple implementations of each.
Second, you make a bizarre reference to "fad[ing] away like Pascal." Why do you think Pascal faded? I'll give a hint: It had nothing to do with being proprietary. At best that reference is a non sequitur, at worst it demonstrates more confusion on your part.
Something being proprietary means that it is owned. It means "relating to an owner or ownership"; "of, relating to, or characteristic of an owner or title holder"; "used, made, or marketed by one having the exclusive legal right"; "privately owned and managed and run as a profit-making organization."
It is literally illegal for me to start marketing The TorstenVL Rust Compiler. Because the language is proprietary.
Trademarks are annoying but I can hardly imagine they're what anyone is worried about when picking a language in this context, they're not what's going to cause a language to disappear.
What you can call something and whether you can legally make the thing or have a permissive license to an existing implementation are two completely unrelated things.
For example, you also can't make a C compiler and name it the "microsoft C compiler" due to your lack of trademark right. Does that mean C is also proprietary?
See also: The most famous open source project is trademarked https://www.linuxfoundation.org/legal/trademark-usage
If you still aren't convinced, you are definitely using a different definition of the word proprietary than everyone else.
> Proprietary software is software that grants its creator, publisher, or other rightsholder or rightsholder partner a legal monopoly by modern copyright and intellectual property law to exclude the recipient from freely sharing the software or modifying it, and—in some cases, as is the case with some patent-encumbered and EULA-bound software—from making use of the software on their own, thereby restricting their freedoms.[1]
> Proprietary software is a subset of non-free software, a term defined in contrast to free and open-source software; non-commercial licenses such as CC BY-NC are not deemed proprietary, but are non-free. Proprietary software may either be closed-source software or source-available software.[1][2]
The Python development team, via the LICENSE file in the GitHub repository, tells me
> All Python releases are Open Source (see https://opensource.org for the Open Source Definition). Historically, most, but not all, Python releases have also been GPL-compatible; the table below summarizes the various releases.
This license is also described on Wikipedia at https://en.wikipedia.org/wiki/Python_License .
Similarly, the reference Rust implementation (https://github.com/rust-lang/rust) is licensed under Apache 2.0 and MIT licenses.
In what sense is FOSS software a kind of non-FOSS software?