This paper presents the development of the AI-based language-learning platform, Revita. It is an intelligent online tutor, developed to support learners of multiple languages, from lower-intermediate toward advanced levels. It has been in pilot use with hundreds of students at several universities, whose feedback and needs shape the development. One of the main emerging features of Revita is the system of linguistic constructs to represent the domain knowledge. The system of constructs is developed in collaboration with experts in language pedagogy. Constructs define the types of exercises, the content of the feedback, and enable detailed modeling and evaluation of learner progress.