Reliably guessing which language the user is writing in can be useful to seamlessly integrate language tools in different contexts such as T295862: View translated messages in Talk pages. In this way the interactions for getting an automatic translation are simplified, not requiring the user to figure out and indicate which is the source language.
An initial iteration providing a basic version of the service is provided in T334465: MinT: Detect language of source content automatically.
Further development, as part of other sub-tasks, will be needed to cover more languages in a reliable way.
Related: T340507: Create a language detection service in LiftWing