Unfortunately, the scope of the case is broader than proprietary languages. It doesn't matter what language you use---if you copy an API (including, IANAL-but-I-assume, recapitulating that API in an entirely different language), you're at risk. This covers a plethora of use-cases.

Can anyone elaborate how this could impact tools such as GNU Octave? It would fall under this classification, right?

I would like to know if it impacts mathematics and scientific libraries for python that I use in lieu of MatLab.

if Octave shares a bunch of function signatures with MATLAB, yes, it would be at risk for a copyright claim.

what about instruction "signatures"? i.e. the machine instruction sets? every compiler is potentially performing millions of counts of copyright and or license violations on a large scale?

I suspect that the implementation of any API would come under the shadow of this ruling. For instance, replacing a proprietary product with an open-source variant that has the same API would also be illegal or subject to licensing fees.

In my opinion, this will be bigger than proprietary languages and cover drop-in replacements for pretty much any proprietary code, thus providing yet another way to lock customers into a particular product.

Yeah, using an ISO standard language (https://en.wikipedia.org/wiki/Category:Programming_languages...) avoids this problem. There a spec is specifically produced so that there can be multiple implementations.

But regardless of what language you use, this legal precedent is saying if you, for example, copy Twitter's RESTful API to create your own messaging service where all the existing Twitter clients can change a few constants and adopt you as a target, Twitter could sue. That's not something ISO protects against (unless there's an international standard for asymmetric message services you can design against).

What is a proprietary language to you? Java? Then you will have a hard time using many languages since it is open source and copyrighted by its owner like many languages.

A proprietary language in this context is one that does not have an ISO standard.

Luckily, Java is fully open source now, with OpenJDK. I concur though with never using proprietary languages (which is a part of me not using C#--of course, that is now also open source, but C# is still kinda bad on Linux afaik).

Edit: _sigh_ The downvotes without any reason hurt. See [1]. Since OpenJDK is GPL v2, and since TCK tests have been open sourced and Java EE given to Eclipse, I would call the Java ecosystem fully open source. Feel free to correct me if I'm wrong of course.

[1] https://openjdk.java.net/legal/gplv2+ce.html

OpenJDK is an open source implementation of the Java API. However the question here is about the openness of the Java API itself, not any particular implementation of it. Google already uses a totally separate implementation which is different from both Oracle Java AND OpenJDK.

Does that make Java/OpenJDK any less open source? If I take OpenJDK and start selling it, how could Oracle get me if OpenJDK is GPL?

We're not talking about re-implementing the JDK/JRE spec; the author mentioned he/she wants to use only open source languages...