Image: Pieter Bruegel the Elder. De “Weinig” Toren van Babel. [The “Little” Tower of Babel]. Oil on wood. c. 1563.
It is, of course, extremely difficult to predict which items of vocabulary a learner will know, even if we have a fairly accurate idea of their level. Personal interests play a significant part, so, for example, some people at even a low level will have no problem with ‘cannabis’, ‘stoned’ and ‘high’, even if these are low frequency. First language, however, is a reasonably reliable indicator as cognates can be expected to be easy. A French speaker will have no problem with ‘appreciate’, ‘unique’ and ‘symptom’. A recommendation engine that can meaningfully personalize vocabulary suggestions will, at the very least, need to consider cognates.