Japanese and Korean are actually not phylogenically related. In fact they’re both generally considered to be language isolates, which means they don’t seem to be related to any other languages. This may not be strictly true and there are proposals that there could be a link through a hypothetical language family called Altaic—they’re usually considered unrelated, though.
The “difficulty” of learning any particular language in relation to another is a bit of a meaningless comparison in my opinion. Ultimately, it appears to be the case that all human languages operate according to a common underlying linguistic mechanism—the seeming differences between particular languages are in fact probably actually just different surface realizations of the underlying mechanism with particular options twiddled in various combinations coupled with an idiosyncratic lexicon (i.e., a relatively unique but ultimately arbitrary vocabulary). The nature of the underlying mechanism is the subject of debate, as well as the list of the particular options available, but it seems pretty clear that all languages are utilizing a common underlying cognitive architecture.
That being said, it would seem to me if you define a relative acquisition difficulty metric as something like
D(x, y) = a*O(x, y) + b*P(x, y) + c*L(y) - d*C(x, y)
O(x, y) = cardinality of the set of differences between the language “settings” of x and y
P(x, y) = cardinality of the set of phonemic distinctions present in y but not x
L(x) = cardinality of the lexicon of x
C(x, y) = cardinality of the set of cognates shared by the lexicons of x and y
where
a,
b,
c, and
d are scaling factors, then that might give you some semi-reasonable rough correlation in the form of a “distance” between two languages along various axes. The exact definition of
O(
x,
y) is far from clear, however. And even just having knocked this out I’m realizing that there’s almost an infinite number of other things you could throw in here that you would arguably need to know to claim to have “learned” a language: phonological processes, knowledge of and relative productivity of morphological processes, pragmatics, etc. This is why I’m of the mind that any such measure is of dubious utility, because you would need to take into account a lot of factors—most of which we still don’t fully understand.
There’s probably work done on this somewhere but second language acquisition isn’t my area of expertise.
The “difficulty” of learning any particular language in relation to another is a bit of a meaningless comparison in my opinion. Ultimately, it appears to be the case that all human languages operate according to a common underlying linguistic mechanism—the seeming differences between particular languages are in fact probably actually just different surface realizations of the underlying mechanism with particular options twiddled in various combinations coupled with an idiosyncratic lexicon (i.e., a relatively unique but ultimately arbitrary vocabulary). The nature of the underlying mechanism is the subject of debate, as well as the list of the particular options available, but it seems pretty clear that all languages are utilizing a common underlying cognitive architecture.
That being said, it would seem to me if you define a relative acquisition difficulty metric as something like
where a, b, c, and d are scaling factors, then that might give you some semi-reasonable rough correlation in the form of a “distance” between two languages along various axes. The exact definition of O(x, y) is far from clear, however. And even just having knocked this out I’m realizing that there’s almost an infinite number of other things you could throw in here that you would arguably need to know to claim to have “learned” a language: phonological processes, knowledge of and relative productivity of morphological processes, pragmatics, etc. This is why I’m of the mind that any such measure is of dubious utility, because you would need to take into account a lot of factors—most of which we still don’t fully understand.There’s probably work done on this somewhere but second language acquisition isn’t my area of expertise.