The Diffusion of West Germanic diphthongization in Eastern and Central Europe
The diphthongization of long i: and long u: to /ay/ and /aw/ is one of the most characteristic phonological developments in English, Frisian, Dutch, German and Yiddish. The study of similar processes in ongoing chain shifts leads to the conclusion that this pattern takes place in a relatively complex phonological space with contrasting peripheral and non-peripheral areas in both front and back. It is not found at all in North Germanic, but does appear in a number of other Indo-European languages: Czech, Polabian, Kajkavian, Old Prussian, Latvian and Romansh. However, the diphthongization of i: seems to lie outside of the realm of phonological possibilities for other branches of Indo-European, e.g., Greek, where eight vowels merged with an unchanged /i:/.
Closer examination of the cases of diphthongization outside Germanic shows that in every case, the non-Germanic languages have been in intense contact with German as a superstrate language, with resultant German influence and language shift. Czech, for example, shows massive importations of German vocabulary and loan translations over many centuries. The only Romance language involved, Romansh, is geographically isolated by German-speaking communities.
It seems likely, therefore, that the development of a phonological space that permits diphthongization of i: and u: is the result of German contact, and it can be inferred that this relatively rare phenomenon is a distinguishing characteristic of West Germanic. If such general features of phonological space can be diffused , this implies that the wave effects of language contact may have more abstract and pervasive influence on the development of languages.than is currently believed.