This paper introduces the linguistic problems resulting from translations of the bilingual philosophical legacy of Ludwik Fleck (1896–1961). The case of “legitimation” is analysed in detail in accordance with the way it was used by Fleck in his original Polish and German texts. Neither Polish nor English translators recognised Polish “legitymować” and its derivatives nor German “legitimieren” and its derivatives as equivalent expressions; hence, one of the most important Fleckian concepts may be misunderstood.