This paper introduces the Lang-Track-App, a smartphone application and backend system to aid second language acquisition researchers in implementing the experience sampling method (ESM). Surveying research participants with the Lang-Track-App multiple times per day can yield exceptionally rich data that can be analyzed in multiple ways. This article explains the different Lang-Track-App components and describes workflows for both researchers and participants. To demonstrate the step-by-step implementation, we present a recent study of language use in study abroad as a test case in which we used the Lang-Track-App to collect ESM data about frequency and qualitative aspects of language use. A selection of findings from this study highlights the richness of ESM data and how they can be analyzed at various levels. Researchers wishing to use the Lang-Track-App to implement the ESM in their own research can access the full open-source code and documentation via GitHub (https://github.com/HumlabLu).