… instead of “What do you say?”
I am not sure if “What say you?” is even grammatically correct.
The way modern English formulates questions by using “do” as an auxiliary is rather recent in the story of English (~1400-1500).
It is termed “periphrastic do” and a growing number of linguists actually ascribe this evolution to the influence of Celtic languages.
The Germanic word order instead is termed V2 syntax (which means that the verb is always in second position).
In Modern German for instance you would say:
“Was sagst du?”
In Old English (before the Norman invasion) you would say something very similar.
“Hwæt sægst ðū”
The UK legal and judicial system is deeply rooted in the Anglo Norman historical period (1066-1485) and therefore predates the “periphrastic do” evolution (~1400-1500).
This is yet another manifestation of the marked conservatism of this institution.