How widespread among languages the usage of the word for “where” as a general relative pronoun (meaning persons or objects)?

That would be the standard modern Greek relativiser I did my PhD on, in fact.

Add Hebrew ašer > še, Bulgarian deto.

Anon (you didn’t need to Anon this time, Anon), I can rule out Albanian: që in standard Albanian, çë in Arvanitika are not locative.