The language Occidental, later Interlingue, is a planned language created by the Baltogerman naval officer and teacher Edgar de Wahl and published in 1922.
Occidental is devised with great care to ensure that many of its derived word forms 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 is 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 exceptionally popular in Europe during the 15 years before World War II, and it is believed that it was at its height the fourth most popular planned language, after Volapük, Esperanto and perhaps Ido in order of appearance.
But some have believed that its intentional emphasis on European forms coupled with a somewhat Eurocentric philosophy espoused by several of its leading lights hindered its spread elsewhere. Yet, Occidental gained adherents in many nations including Asian nations. Before WWII it had grown to become the second largest IAL in numbers of adherents, after Esperanto. Esperantists at the time claimed Occidental had at least 2,000,000 adherents. Also, the majority of the Ido adherents took up Occidental in place of Ido. Cosmoglotta, Oct. 1928, Num. 53(10), p. 142, 149-152, Ido-Congress in Zürich.
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. However, today with the emergence of the Internet, Occidental is once again increasing in popularity.

Example texts

Indo-European languages
Resume de gramatica de Interlingue (Occidental) in Interlingue, (e demonstration de leibilita!)
Li Europan lingues
Interlingua