A dictionary is a reference book that lists the words of one or more languages, usually in alphabetical order, along with information about their spe… - David Crystal

There is little scientific data on the point, but evidently people do speak to themselves.

During the later part of the 19th century, it was believed that a sound of change affected the whole of a language simultaneously: one sound system would smoothly develop into the next, and all words which contained a particular sound would be affected in the same way.

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.