Are Dutch and French languages miscible?

French and Dutch are two languages of different origins (Germanic vs. Romance) that coexist within the nation-state of Belgium. While they are mostly segregated throughout the Belgian territory, in Brussels they reach an actual cohabitation with a relevant bilingual population. The dominant language in Brussels shifted from Dutch to French during the late XIX century in a process known as the Francization of Brussels. The fractions of speakers of each language and of bilinguals over that time were recorded periodically until political tensions ended the censuses in the country. This relevant linguistic shift has been the object of sociopolitical studies, but the available empirical data have never before been analyzed using a theoretical mathematical model that would allow us to quantify causal factors behind the observed dynamics. Here we carry out such study for the first time, measuring effective values of perceived interlinguistic similarity and language prestige, among others. This modeling and quantification allows us to speculate about possible trajectories of fractions of speakers over time—specifically, whether Dutch and French tend to be languages that can coexist in the long term. We find that there is an overall tendency of both tongues to grow segregated over time, suggesting, in physics terms, that Dutch and French are not miscible. The scenarios that would allow for language coexistence would often see a starkly dominating language. Notwithstanding, we also discuss the costs of attempting to sustain the cohabitation despite a natural tendency to the contraryOpen Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. JM is part of iMATUS - USC and of the Program for Consolidation of Research Units of Competitive Reference (GRC), supported by the Xunta de Galicia. LFS has received funding from the Spanish National Research Council (CSIC) and the Spanish Department for Science and Innovation (MICINN) through a Juan de la Cierva Fellowship (IJC2018-036694-I). The Spanish MICINN, through grant SEV 2017-0712, has also funded the “Severo Ochoa” Centers of Excellence distinction for the CNB (host institution for LFS during the development of this work)S