This thesis describes the processes of mother tongue and second language acquisition and learning. Mother tongue acquisition is influenced by factors such as gender, number of siblings, parent-child relationship, language patterns, linguistic environment, social status of parents, etc., whereas foreign language proficiency is influenced by age, language learning strategies, motivation, etc. Due to globalization, rapid technological advancements and considerable migrations that are taking place today, mastering multiple languages has become a necessity, so it is not surprising that bilingualism is becoming more common. Bilingualism is the ability to use different languages and dialects in a variety of situations for different purposes and interlocutors, and has become a desirable phenomenon across Europe. With the English language ruling the economic, scientific, cultural and social aspects of our lives, other foreign languages are also becoming important. In this master's thesis, the emphasis is set to two Slavic languages, Croatian and Polish, which have been connected since the arrival of the Poles in today's Croatia. Later on, strong Polish-Croatian ties were developed, influenced by several historical events, followed by the emphasizing their national languages and ensuring their use as official languages in these countries. Croatian and Polish are compared on a phonetic-phonological and morpho-syntactic level. Based on the expert knowledge about these two languages, various tasks have been introduced to improve the linguistic development of Polish-Croatian bilingual children. Foreign language teachers can focus more on students so that they become more relaxed and successful in communicating in a foreign language. This master's thesis strives to give teachers ideas of how to enhance classes and make them interesting and unpredictable.