Occidental

Occidental was a naturalistic planned language which was devised with great care to ensure that many of its derived word forms would reflect the similar forms common to a number of Western European languages. This was done through application of de Wahl's rule[?] which is actually a small set of rules for converting verb infinitives into derived nouns and adjectives. The result was a language relatively easy to understand at first sight for individuals acquainted with several Western European languages. Coupled with a simplified grammar, this made Occidental moderately popular in Europe during the decade and a half before World War II, and it is believed that it was at its height the fourth most popular planned language, after Esperanto, Volapük and perhaps Ido, though its intentional emphasis on European forms coupled with a somewhat Eurocentric philosophy espoused by several of its leading lights hindered its spread elsewhere.

Occidental survived World War II, undergoing a name change to Interlingue, but gradually faded into insignificance following the appearance of a competing naturalistic project Interlingua in the early 1950s.