Programming is a Science,and the Programming Language in question becomes an implementation of such a Science.
Assuming the position is for a Python programming beginner, they must have sound knowledge of the difference between an interpreter and a compiler, and you should also throw in a question of how Java works , since it brings in the concept of byte code based virtual machines. I myself am a Perl programmer learning Python, so the first thing i try to learn as a Python programmer is how its data structures are implemented.
The key , in my opinion , to learning the nuances of a language, is a thorough understanding of its data structures(arrays, hashes, trees) , and hence by necessity its data types.
Concepts likes OOPS, and classes, are programming language independent, but based on the need of the project, one can always throw in a few questions to ascertain their knowledge of how Classes, Objects are represented in the language, and their degree of proficiency can also be gauged by nuances like how is poly morphism implemented ,and how basic memory management is done in the language.
As a Javascript programmer, a basic understanding of the DOM is a backbone to being a good JS programmer. By current web programming standards , there is an over dependence on Jquery and other libraries, so one can indulge JS programmers on key questions of writing performant, secure code that they need to be aware of.
Good debugging skills are essential for any JS programmer, as unlike Java it is not an easy language to quickly understand others code. So if you are tasked with being a JS programmer in an organization, you must be able to debug code yourself and figure out constructive solutions to problems you / others are facing.