How Are Multilingual Systems Constructed: Characterizing Language Use and Selection in Open-Source Multilingual Software.

For many years now, modern software is known to be developed in multiple languages (hence termed as multilingual or multi-language software). Yet to this date we still only have very limited knowledge about how multilingual software systems are constructed. For instance, it is not yet really clear how different languages are used, selected together, and why they have been so in multilingual software development. Given the fact that using multiple languages in a single software project has become a norm, understanding language use and selection (i.e, language profile ) as a basic element of the multilingual construction in contemporary software engineering is an essential first step. In this paper, we set out to fill this gap with a large-scale characterization study on language use and selection in open-source multilingual software. We start with presenting an updated overview of language use in 7,113 GitHub projects spanning five past years by characterizing overall statistics of language profiles, followed by a deeper look into the functionality relevance/justification of language selection in these projects through association rule mining. We proceed with an evolutionary characterization of 1,000 GitHub projects for each of 10 past years to provide a longitudinal view of how language use and selection have changed over the years, as well as how the association between functionality and language selection has been evolving. Among many other findings, our study revealed a growing trend of using 3 to 5 languages in one multilingual software project and noticeable stableness of top language selections. We found a non-trivial association between language selection and certain functionality domains, which was less stable than that with individual languages over time. In a historical context, we also have observed major shifts in these characteristics of multilingual systems both in contrast to earlier peer studies and along the evolutionary timeline. Our findings offer essential knowledge on the multilingual construction in modern software development. Based on our results, we also provide insights and actionable suggestions for both researchers and developers of multilingual systems.