Before being release to the public, has the syntax of any programming language undergone usability testing? If so, what kinds of testing was performed, what were the results, and what impact did the testing results have on the design of the language?
From Robert Harvey's comment I found this interesting quote from C# designer Anders Hejlsberg:
Anders Hejlsberg: Most of it was actually usability studies of IDE features. We might ask, "Can people understand that they right click to do this or that?" We did some usability studies for the pure language syntax itself—I think we did some with properties and events, for example—but it was not necessary really.
I don't think that you get as high a yield from usability studies for language features as for IDE features. IDEs are very interactive. You can watch users right click menu items and get good feedback. For programming languages, the question is more, "Is it conceptually understandable?" That's done very well by having a customer advisory councils, sounding boards. You want places where you can say, "Here's what we're thinking about doing for this particular new feature. What do you all think?" And you actually urge them to shoot as many holes in it as possible, because you'd much rather know before you put in the feature than after. So unless a language feature is a complete slam dunk, we tend to make use of those kinds of sounding boards.
If by language usability you mean how the syntax and semantics of a programming language influences productivity, some studies have been conducted to evaluate the average productivity (time needed to implement a given program and quality of the solution) of programmers using several languages.
You can find some information (with citations of further articles) on this page. The page has been written by a Lisp programmer so one might argue that it is biased (i.e. that it presents information that is favorable to Lisp). Still I find it interesting to look at how such experiments can be conducted and what one can try to measure. Also, the cited articles may provide more links to related studies.
One could argue that a "usability test" of Fortran II leaded to a complete new language: BASIC, which was designed to be more usable (especially for beginners) than its predecessor. If you want to know more about the origins and design goals, there is a whole chapter in this book concerning that topic.