During intercultural communication, it is crucial to interpret correctly and to use appropriately foreign idioms which are culturally marked and reflect linguistic and cultural identity of a speech community. Interlocutors should be aware of the cultural and historical precedents that gave rise to the primary image underlying idiomatic expressions and thus created their unique phraseological worldview. The aim of the research is to find out what is a better predictor of correct idiom interpretation - degree of proficiency in a foreign language or degree of genealogical kinship between the native and foreign languages. The topicality of the research is justified by the need for a deeper understanding of linguistic and cultural identity of native and foreign-language speakers, with a view to facilitate and enhance cross-cultural communication. The working hypothesis is that due to the close genealogical kinship between Russian and Bulgarian and the users’ advanced level of English, the number of correctly interpreted idioms may vary within a statistically significant medium range. The total sample comprises 5000 idioms (2500 English and 2500 Bulgarian ones). The subsample used in the experiment comprises 60 idioms (30 English and 30 Bulgarian ones) selected from ‘The Oxford Dictionary of Idioms’ and ‘Nov fraseologichen rechnik na bylgarskiya jezik’ (‘Нов фразеологичен речник на българския език’) by means of stratified systematic sampling. The main methods used in the research include (1) comparative linguistic and cultural analyses, (2) scientific experimentation, (3) systematic and stratified sampling, and (4) a paired t-test. The experimental research and the paired t-test have proved our hypothesis and demonstrated that Russian participants correctly decode more Bulgarian than English idioms, with intergroup variation being statistically significant. Research findings have implications for cultural linguistics. Since translation loans (calques), isomorphic idioms, and idioms dating back to a common source are interpreted more quickly and more accurately than idioms which contain unique or culturally-loaded elements, such as old-fashioned words or proper names, access to cultural precedents that served as prototypes of set expressions contributes to a more seamless code-switching and enables communicants to penetrate deeper the mentality of a specific linguacultural community and thus become aware of the variability of cultural cognition and conceptualisation.