Capturing the diversity of multilingual societies Multilingualism is a pervasive phenomenon worldwide with around 6000 spoken languages in 200 nations. In almost every country, the presence of more than one language naturally leads to speech communities of different sizes, with many individuals belonging to these communities using two or more of them. This occurs independently of the official status and the educational prevalence of those languages. Therefore, understanding what mechanisms lead to language extinction and which of them might enable coexistence is key to preserve cultural diversity.  To this end, the IFISC (UIB-CSIC) researchers collected geolocalized Twitter posts from 16 countries between 2015 and 2019 with the aim of exploring spatial patterns of language coexistence in multilingual societies. This allowed them to determine the extent of spatial segregation, defined as the difference in how speakers of a language group are distributed compared to the total population. The researchers observed countries or regions with a high segregation of monolingual communities, such as Switzerland, while others, such as Catalonia, are a paradigmatic example of the opposite behavior, characterized by a high mixing among groups.  The work also highlights the fact that in certain bilingual societies monolingual speakers of one language are virtually extinct, as is the case in Catalonia, Quebec or the Basque Country. In these situations, bilinguals act as a linguistic reservoir for the endangered language, keeping it alive for long periods of time. The proposed mathematical model takes into account the cultural attachment of bilinguals for one of the two languages. This preference can act as a defense mechanism of the endangered language since its use by bilingual speakers may suffice to save it despite a possibly lower social prestige of this language. The researchers find that this novel preference parameter is critical to determine whether languages can coexist or not.  Overall, these findings shed light on the role of the heterogeneous speech communities in multilingual societies and they may help shape the objectives of language planning policies in countries where accelerated changes are threatening cultural diversity.   Capturing the diversity of multilingual societies. Louf, Thomas; Sánchez, David; Ramasco José J. Physical Review Research 3, 043146 (1-11) (2021)El Diari de la UIB http://ifisc.uib-csic.es/en/news/capturing-diversity-multilingual-societies/