Reserva de ley is the principle under which the Spanish Constitution specifies that certain areas of the law may only be regulated by primary legislation in the Spanish Parliament, the Cortes Generales. According to Wikipedia, this constraint or guarantee is found in at least 70 articles of the Spanish Constitution.
Although the term reserva de ley itself does not appear anywhere in the Constitution, it is a recognised principle in Spanish law (often referred to as principio de reserva de ley) and is apparently of German origin. (See this post by Margaret Marks on Transblawg for comments on the German term and this post.)
While similar principles exist in other civil-law jurisdictions such as Germany, Sweden, France and Venezuela, the concept does not appear to be enshrined as a specific principle anywhere in the common-law world.
So, here we have a culture-specific term, very common in legal translation and what I was referring to in this post on the merits of translating such terms descriptively against using linguistic or literal strategies.
A descriptive translation for reserva de ley in a phrase such as XXXX es materia sometida a reserva de ley might read something like “[matter/area/whatever it is] that must be regulated in the Spanish Parliament/by primary legislation”, which is more or less what I used when I had to translate this term recently.
Linguistic*, in this case, literal, solution
A linguistic-focused and literal translation might be something like “legal reserve”, “legal reservation” or “Under the ‘Reserva de Ley’ principle”, which are some of the examples you can find around the Web — both in translations and in term resources.
What makes more sense?
My biggest problem with a literal translation here is that “legal reserve/reservation” is probably going to mean very little to our potential reader: if we’re lucky, an English-speaking lawyer, but maybe the Kansas City milkman (see this post), in which case we can take little for granted as far as assumptions about field-specific knowledge go.
Maybe the reader will be a lawyer trained in a system with a similar principle, but that’s not the safest assumption to make.
The concept might be familiar in English in comparative law circles and may be used in the literature on the civil-law tradition, but I think this would count out most of the cases for which it might come up in translation and even then would require some kind of description.
And of course, whenever there’s a risk a term may not be understood, it may also be misunderstood and confused with some other concept if sufficient help is not provided for the reader.
In this case, description seems to be the safest and best bet if you want your reader to understand what reserva de ley means.
When I had to translate this term recently (in an academic paper), I was always pretty sure a descriptive solution was the way to go. But, as it was a principle that the reader might want to research themselves, I considered leaving the term in Spanish in brackets but didn’t in the end as in this case the reader didn’t need to know such a principle existed (although, even without mentioning it, this is the conclusion the reader would have to come to if they thought about it); the important thing was to know that the area of the law in question had to be regulated by primary legislation.
I can imagine cases in academic papers where you might need to introduce the term (in this case, the principle) to the reader where you might use a description and a literal solution, but this would definitely be the exception, and in all cases I would include a description alongside.
Descriptive solution found in an EU document:
It also requires this right to be defined by an Act of Parliament (reserva de ley ordinaria)
Definitions in Spanish:
Materias que, según la Constitución, han de ser reguladas por una norma con rango ley aprobada por los representantes del pueblo en Cortes, no pudiendo el poder ejecutivo normar aquéllas. La reserva de ley puede ser: 1) ordinaria, y 2) orgánica. Por otro lado, la reserva relativa de ley permite que, excepcionalmente y cuando esté expresamente autorizado, algunas de estas materias puedan ser reguladas por el Gobierno.
La reserva de ley o dominio legal es el conjunto de materias que de manera exclusiva la Constitución entrega al ámbito de potestades del legislador, excluyendo de su ámbito la intervención de otros poderes del estado. […] De acuerdo a la Constitución Española de 1978 existen reservas de Ley en una setentena de artículos. En ocasiones dicha reserva es de Ley Orgánica (como los Derechos Fundamentales, los estatutos de autonomía y el régimen electoral general) y en otras ocasiones de Ley Ordinaria.
La Constitución reserva determinadas materias para ser reguladas, de una manera más o menos completa, por la ley: es lo que se denomina reserva de ley.
UNED material (in Spanish) on reserva de ley:
* Note to translation theorists: Sorry if I’m butchering the concept of “linguistic translation” here. In this case it may only amount to literal translation, but “linguistic” seems to fit better in a broader sense.