The structural closeness of languages to each other has often been thought to be an important factor in FLL (foreign language learning). If the L2 [the foreign language] is structurally similar to the L1 [the original language], it is claimed, learning should be easier than in cases where the L2 is very different. However, it is not possible to correlate linguistic difference and learning difficulty in any straightforward way, and even the basic task of quantifying linguistic difference proves to be highly complex, because of the many variables involved.

p. 371 - The Cambridge Encyclopedia of the English Language, 1987