The interference of languages in minds of multilingual speakers, mastering several foreign languages in addition to their mother tongue, is inevitable. Much is still unknown about differences in the innate ability to learn a foreign language and its relationship to native language proficiency. Being a multiethnic and multilingual country, Kazakhstan supports the policy of trinity of languages - Kazakh, Russian and English. The purpose of this research is to study the language performance of the Kazakhstani trilingual youth exposing language deviations in speech practices in three languages - Kazakh, Russian and English. The research method used in the study comprised interviews, conducted among the representatives of the Kazakhstani youth. Sixty-six students of K. Zhubanov Aktobe Regional University (Aktobe, the Republic of Kazakhstan) aged 18-29 years participated in the study. The collected interview data were used to compare the respondents' performance in the Kazakh, Russian and English languages, to describe the specifics of their language processes in the abovementioned languages, and to identify language inconsistencies, if there were any, in their communication. Code-switching, code-mixing, pauses, fillers, and context mismatch were among the deviations examined. The study results demonstrated that language deviations in speech and communication are a usual practice for the representatives of the Kazakhstani youth. Language deviations such as code-switching, code-mixing, pauses, fillers and context mismatch were observed in the speech of Kazakhstani multilingual youth. The most common among language deviations in the speech performance of Kazakhstani youth were code-switching (24.4%) and code-mixing (41.5%) in Kazakh, fillers (65.5%) and code-mixing (13.8%) in Russian, pauses (29.2%) and fillers (29.2%) in English. The scientific significance of the study implies the defining of language deviations occurring in speech practices of Kazakhstani multilingual youth in Kazakh, Russian and English languages. The practical significance of research lies in the fact that the study findings will contribute to a better understanding of the performance of different languages in a multilingual's mind, especially in the context of adopting the trilingual policy in Kazakhstan, as well as to an effective communication. [ABSTRACT FROM AUTHOR]