This constraint type would apply to properties whose value is a monolingual string.
The “single value per language” constraint would allow one value per language of the string, in effect using the language as the "separator" qualifier. Ideally, it would ignore deprecated values in making the calculation.
Problem to be solved: well-known paintings, dual-language books, and taxa common names cannot be modeled correctly in Wikidata without constraint violations or exceptions to the constraint.
Typical use case: Q12418 has title "Mona Lisa" in EN, "La Joconde" in FR, "La Gioconda" in IT, and so on.