Meeting the challenges of competitiveness and professional mobility as well as successful carrying out effective communication in the professional sphere implies the need for university students to master two or more foreign languages. Current linguistic situation characterized by overall spread of multilingualism has sparked a growing interest as to how FL1 (first foreign language) could possibly interfere with the learning of FL2 (second foreign language). In spite of extensive research dealing with this problem, the interaction between the two foreign languages in the context of teaching and learning has not been thoroughly addressed yet. This paper focuses on the lexical interference that occurs when the professional English terminology in the field of International Relations is studied in the context of language contacts between FL1 (French) and FL2 (English). The long-standing teaching practice has inspired the author to scrutinize such interference and to put forward a hypothesis that when the English language is studied as FL2 after French, both lexical interference and positive transfer will inevitably occur, and the source of both these phenomena when studying the International terminology will be the French language. Typical students' errors due to the language interference depend on the linguistic experience gained with FL1. The research methodology included questioning and testing of students who studied English as FL2, detecting and classifying their typical lexical errors. To find out the range of potential lexical interference of the contacting languages a comparative study of the professional diplomatic terminology of the two languages (English and French) was performed followed by a pilot study where actual occurrences of such interference, i.e., students' lexical errors were detected, analysed and systematized, and their sources were identified. The participants were Russian-speaking students seeking a Bachelor degree in "Theory and History of International Relations" in the Peoples' Friendship University of Russia and studying English as their FL2. The results of the study confirm that the level of lexical interference in professional diplomatic terminology learning is extremely high where the French language is a primary source. The results demonstrate that teaching English for Special Purposes after French as FL1 is a task that requires thorough consideration of everything that could possibly aggravate the learning process. Typical errors of students will occur due to their former linguistic experience and, more importantly, to this very language interference between their FL1 and FL2. The research makes it clear that the main source of errors is completely or partially assimilated cognates. The author comes to the conclusion that a special systematic approach should be developed and presents a set of exercises which would make it clear where and why a student chooses wrong terms and how to avoid this in teaching English as FL2.