Conceptual clarity refers to a deep and clear understanding of a concept.
This is most easily achieved when the language of instruction is the one the learner is most comfortable and fluent in.
For most students, especially in the early years, this is their mother tongue.
When teaching occurs in the mother tongue, the student does not have the cognitive load of simultaneously learning a new language and a new concept. They can directly grasp the idea being taught.
While regional and national languages are important, the mother tongue provides the strongest foundation for initial conceptual understanding.