Peter Müller has a not-very-long post on this term:
bq. Judex a quo: Gericht, welches die Entscheidung erlassen hat (Prozeßgericht)
Judex ad quem: Nächsthöheres Gericht
I have had to translate this into English before. I think it was in a Bundesverfassungsgericht decision and it was suggested I keep it. The terms are in Black’s Law Dictionary, but they are described as ‘Civil law’, i.e. not English or U.S., terms. I think it needs defining in English.