There’s a term in German constitutional law, Gesetzesvorbehalt, literally (reservation/requirement of a statute).
On Legally Yours, Rob Lunn discusses the equivalent Spanish concept. How to translate “reserva de ley” into English (using a descriptive strategy).
In my database I find a suggestion to translate the German term as ‘constitutional requirement of the specific enactment of a statute’ (because secondary legislation is not enough).
It is apparently sometimes translated as ‘legal reservation’ or ‘reservation of law’, which doesn’t convey the meaning at all.
The word Vorbehalt is often a problem. If you translate it as ‘reservation’, you are using a word that’s less usual in legal English than Vorbehalt is in legal German.
I prefer ‘requirement’.
There’s a discussion of the term on LEO (quite useful in parts, but I particularly enjoyed the comment ‘I actually discussed that topic with a common lawyer. He completely ignored that concept’ with its interesting use of ‘ignored’).
I’ve apparently had to translate quite a few words with ‘Vorbehalt’ as part: Änderungsvorbehalt, Beamtenvorbehalt/Funktionsvorbehalt, Eigentumsvorbehalt (reservation/retention of title), Einwilligungsvorbehalt, Erlaubnisvorbehalt, Identitätsvorbehalt, Kontokorrentvorbehalt, Liefervorbehalt, Parlamentsvorbehalt (another term for Gesetzesvorbehalt), Progressionsvorbehalt, and several more.
I can’t quite agree with Rob that this is such a culture-specific term (see Things I learnt from a journo about translating culture-specific terms: (1) Description trumps linguistic solutions), but OK, it is not a concept that applies to UK constitutional law. I would definitely use the definition here, and I might not add the German in brackets.