The article examines the interaction of the German and Russian languages in the speech of bilingual Germans who were born and live in a Russian-speaking environment in the Vyatka region of Russia. This task involves the study of their usage, interference (including phonetic, lexical and syntactic assimilation), as well as borrowings and code-switching. The dialects of the Russian Germans of the Vyatka region have a status of migrant and belong to a category of vanishing supraregional linguistic entities. The function of this variety of language is to provide a link between the native German language of the immigrants (L1) and Russian, their major surrounding language (L2). In addition, the German language of the Vyatka region reflects new linguistic contacts caused by multiple forced migrations during the Second World War. As a result of these mass relocations, some new processes in interaction of dialects arose, not observed in their mother colonies. The resulting variety of usage can be referred to as a German-Russian interlanguage. Before the World War II, all German dialects for more than two centuries have been confined in Russia to enclaves (or dialect islands). After mass deportations, they transformed both geographically and, for certain dialects, in terms of social composition. Taking this into account, the study of their interactions acquires greater importance for understanding similar processes associated with modern intercultural language contacts, in general. These changes in the language environment boosted linguistic interference at all levels; they also account for the tendency to bilingual behavior common both for speakers of standard and dialectal German. Our analysis of these processes is based on the interviews with bilingual Germans of the Vyatka region of Russia recorded during dialectological trips to this enclave. The study identifies and describes phonetic interactions (both segmental and supersegmental), morphological and syntactic interferences in the Russian and German speech of the Germans of this dialect island. All these processes in L1 and L2, as well as their distortion and mixing, are typical for the mechanisms governing their bilingual performance as well as the degree of its stability.