This article considers the usage of native and foreign languages in the multinational community of Russian diplomats in the second half of the eighteenth century, especially by the immigrants from the Ostsee (Baltic) provinces of the Russian Empire annexed as a result of the Great Northern War of 1700–1721. Incorporating the approaches of new diplomatic history that applies the methods of sociolinguistics and studies, among others, the national, social and cultural identities of international actors, the author examines language practices and language competences of Swedes Karl Gustav (Karl Matveevich) and Johann Matthias (Ivan Matveevich) Simolin in the context of their career strategies. The study refers to their correspondence with Russian monarchs and the leadership of the Collegium of Foreign Affairs over the 1740s–1780s kept in the Archive of Foreign Policy of the Russian Empire and the Russian State Archive of Ancient Documents. The reconstruction of both brothers’ career paths reveals that they used similar language competences they initially had in different ways. Karl Gustav, the elder brother, translator, and polyglot, was responsible for correspondence in German in the Collegium of Foreign Affairs for many years. However, when he received an appointment to Mitau, his only diplomatic post, in 1758, he began to conduct official correspondence in the Russian language. Johann Matthias, the younger brother, spent most of his life abroad and, starting from 1758, held diplomatic posts in Regensburg, Copenhagen, Stockholm, London, and Paris. German that he spoke as a native speaker and French were his main languages of correspondence with the Russian monarchs and the leadership of the Collegium of Foreign Affairs. French gradually replaced German in documents of all types as the language of international and court communication. In addition, Simolin understood the Russian language perfectly, which, from the mid‑1770s, spared the Collegium from having to translate rescripts sent to him on behalf of the empress. The analysis of both brothers’ language practices demonstrates that despite the introduction of French into the internal correspondence of the Collegium of Foreign Affairs in the second half of the eighteenth century, the new subjects of the Russian Empire found it necessary to learn Russian to gain promotion and become part of the Russian imperial elite.