This article provides qualitative and quantitative data to establish the effectiveness of the polylingual approach application in the second foreign language classroom. The relevance of the research is associated with the need to develop guidelines and teaching materials for polylingual classrooms. In the context of classroom polylingualism, one of the key elements is the phenomenon of the cross-language interference, which generates problems in intercultural communication at the discourse level. The studies of interference conducted so far are mainly dedicated to its phonetic and lexical subspecies, attention is drawn to the morphosyntactic aspect of teaching a second foreign language. The novelty of the research is that for the first time a purposeful study of cross-language interference as a pedagogical resource in teaching several foreign languages is carried out, the expediency of using the polylingual approach is substantiated. Previous research in the field has shown that the polylingual approach might be applied in a polylingual classroom studying foreign languages, but has neither established the conditions and principles, nor proposed the corresponding teaching materials. This study substantiates the need for the implementation of the polylingual approach based on taking into account cross-language interference; the rationale is determined by the solution of professional tasks which students of the humanities are facing when acquiring both universal and professional skills, the visible presence of positive and negative cross-language interference in the grammatical aspect of the second foreign language. Due to the fact that there is no single definition of cross-language interference, we have developed one that is relevant for our research, emphasizing that interference can be both an independent transfer of the norms of one language to another, and the result of code-switching. During the first term we surveyed and observed the target audience, pursuing the aim to collect and classify the data referring to the typical mistakes made in the classroom caused by cross-language interference. A total of 56 1st- and 2nd-year students and 43 professors participated in the research. The students took part in the trial training of the polylingual approach (two terms), which contained the application of the designed teaching and learning materials and a block of module assessment and final assessment, evaluating progress in studies and the decline of the mistakes marked on the 1st stage. The professors commented on their experience of the application of the approach and their wish to apply it in general. The results reveal that the polylingual approach may be successfully implemented in the chosen context if the proposed model and algorithm are followed. The findings suggest that the polylingual model is universal and can be further used in any polylingual classroom of the second foreign language as a basis for its unique educational context.