This article explores cross-linguistic lexical influence (CLI), i.e. idiosyncratic borrowings and creations based on other languages, in texts written by pupils learning French and English as foreign languages, and whose language of instruction is German. The study, carried out in German-speaking Switzerland, was based on a plurilingual conception of language learning and drew on research on tertiary languages. Two groups of learners at the end of Grade 7 (age 13/14) were compared before and after a reform in the teaching of English. While both groups had been learning French since Grade 5, one group had been learning English before French since Grade 3 (Group 3/5, n = 220) and the other after French since Grade 7 (Group 5/7, n = 94). The L3 French texts of group 3/5 revealed a higher number of idiosyncratic borrowings and creations based on English L2, while the English L3 texts of group 5/7 borrowed more from L2 French and presented more lexical creations that drew on German. The correlations showed that learners who use lexemes with a detectable cross-linguistic influence in one language also tend to do so in the other. The results are interpreted in relation to the educational context, and to the concept of translanguaging.