If a language uses relative pronouns of interrogative form in ad-relative* clauses, then in that language interrogative pronouns are placed in initial position in questions.
Standardized
IF relative pronouns of interrogative form are used in ad-relative clauses, THEN interrogative pronouns are placed in initial position in questions.
* ad-relatives are prenominal […S…NP]NP and postnominal [NP…S…]NP relative clauses, in which S is adjoined to the head NP.