On the basis that polylexicality is a morphological mark that allows us to integrate the notion of equivalence between polylexical and monolexical units into translation, we tried to demonstrate the tangled relation between two essential mechanisms of lexical dynamics, namely those of fixation and derivation. This complementarity, given that it is far from being translated in the same way, is not unproblematic during the passage from one language into another, more particularly from French into Arabic or conversely. It represents an obstacle impossible to circumvent when one is faced with the development of bilingual French-Arabic concordancers.