can you expand? it - http://www.shenlanguage.org/license.html - seems to be intended to let anyone use the language while keeping the language itself under the control off the authors. is that correct? if so, what's the problem?
Not even close to an Open Source license, so calling it "liberal and sound" seems quite misleading. Permits only a very narrow class of derived works ("improving performance, correcting an error, or porting to a new platform"). Requires compliance with "the latest standards for Shen", not just the standard at the time, which makes for a moving target and an effectively revocable license. Cannot incorporate into anything not named "Shen".
The authors can of course choose any license they like, but given the widespread availability of languages with Open Source implementations, I see no reason to give a second look to a language both proprietary and obscure.
I think "liberal" should imply at least open source. After reading the license, it seems I can't incorporate part of the Shen compiler in my own project or add features to it. While the author presents his reasons for these limits, and they may be reasonable, I don't think this makes the license "liberal". Or rather, it's only liberal relative to proprietary licenses; but this is like saying that Chinese government is "liberal" because it affords so many more freedoms than a complete dictatorship.
As far as "sound" goes, it's hard to tell since I'm not a lawyer and I don't think it was ever tested in court. Assuming the license was written by a competent lawyer familiar with the relevant law, it is probably decently sound. If it was not written (or at least edited) by a good lawyer, it becomes somewhat suspect.
The specification itself is copyrightable, but the language it specifies isn't necessarily copyrightable.
I do not know the exact particulars of the Oracle v Google case, but my impression was that the judge ruled that APIs could not be copyrighted, which does not necessarily reflect anything about the language itself. Even more tentatively, I do not think a programming language could be copyrighted, but I would want somebody else to answer that more authoritatively.
Either way, this license is actually about the implementation of Shen. So if you want to write your own compiler from scratch for the same language, you're free to do so. In fact, this is actually mentioned in the FAQ at the end of the license page. That said, beyond copyright, they also control the name "Shen". So you'd be free to implement a language that was mostly, but not completely, a clone of Shen, you would be free to (assuming you do not reuse any of their code), but could not actually call it "Shen".
Anyhow, my knowledge of the law is laughably incomplete and I am clearly not a lawyer, so take everything I've said with a grain of salt.