I have summoned up the courage/energy/folly? to post my suggestion. To summarise what was said in the discussion area, the term "responsabilidad personal subsidiaria" is defined and explained in Article 53 of the Spanish Criminal Code. Here is is again:

"1. 1. Si el condenado no satisficiere, voluntariamente o por vía de apremio, la multa impuesta, quedará sujeto a una responsabilidad personal subsidiaria de un día de privación de libertad por cada dos cuotas diarias no satisfechas, que, tratándose de delitos leves, podrá cumplirse mediante localización permanente. En este caso, no regirá la limitación que en su duración establece el apartado 1 del artículo 37.
También podrá el juez o tribunal, previa conformidad del penado, acordar que la responsabilidad subsidiaria se cumpla mediante trabajos en beneficio de la comunidad. En este caso, cada día de privación de libertad equivaldrá a una jornada de trabajo."
http://noticias.juridicas.com/base_datos/Penal/lo10-1995.l1t...

So although in practice it usually refers to incarceration, the law explicitly provides for the possibility of a non-custodial sanction, either "localización permanente" for "delitos menores" (house arrest, or electronic monitoring) or community service.

Therefore, in principle, to translate it simply as "incarceration" rules out other possibilities which were deliberately ruled in when this article of the CP was revised in 1995. The possibility of alternative non-custodial forms of "responsabilidad personal subsidiaria" was new in 1995; the 1973 CP had nothing like it, though in 1988 the TC had already ruled that that the "régimen de cumplimiento" of CP 1973 Art. 91 was open to judicial discretion: the judge "no necesariamente tiene que configurarla como privación de libertad de ejecución carcelaria, sino que puede optar por formas amortiguadas de cumplimiento como el arresto domiciliario o por modalidades de restricción de derechos de distinta naturaleza". The legislator made this explicit in 1995.

Now, I'm sure that Richard is right to say that it is almost always custodial in practice. And it's also true, apparently, that the possibility of community service has not been used all that much because it's difficult to implement: the system has not really been geared up to it. Nevertheless, the principle of avoiding custodial sentences is an important one and is widely recognised internationally. There's an interesting discussion of this here, in an article on "Las penas leves tras la reforma de 2015":

“la posibilidad de cumplir la responsabilidad personal subsidiaria por impago de multa trabajando en libertad para la comunidad supuso en su día un avance considerable en la lucha contra las penas cortas de prisión. Su introducción en el Código Penal de 1995 fue bien recibida por la doctrina”
https://www.fiscal.es/fiscal/PA_WebApp_SGNTJ_NFIS/descarga/P...

And it's interesting to note, in passing, that Ireland, for example, has been moving strongly in this direction, trying to make incarceration for unpaid fines a last resort:
http://www.citizensinformationboard.ie/downloads/relate/rela...
https://www.irishtimes.com/news/crime-and-law/drop-in-number...
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Sorry about the very long preamble; I've been trying to make the case for seeking an alternative translation. What should it be?

"Subsidiary" is used in some sources, especially in translations, as mentioned in the discussion, and is the official term in the Philippines.
https://www.lawphil.net/statutes/repacts/ra1969/ra_5465_1969...

But I still think the word "subsidiary" is not really appropriate in English; it means "less important than but related or supplementary to something" (Oxford), and I don't think that's the idea; this is neither less important or supplementary.

Richard's suggestion of "secondary penalty" seems to me a good idea. My one reservation is that this term comes with a different sense attached: it's applied to a secondary principal penalty (for example, a fine in addition to imprisonment in the original sentence).

Personally I prefer "alternative", because I think it accurately expresses what's involved: imprisonment, house arrest/electronic monitoring or community service as an alternative to an unpaid fine. There's an interesting article here from the California Law Review of 1969 on "Fines, Imprisonment, and the Poor: Thirty Dollars or Thirty Days". It repeatedly uses the term "alternative punishment":

"Part II examines the penology of imprisonment both as a means of collection and as an alternative punishment to the fine [...]"
https://scholarship.law.berkeley.edu/cgi/viewcontent.cgi?ref...

It's true that "alternative" also comes with baggage; it tends to mean alternative to imprisonment (suspended sentences, community service, semi-liberty). See Article 39 here on p. 113 (p. 16 of file):
https://www.usip.org/sites/default/files/MC1/MC1-Part1Sectio...

Since non-custodial "responsabilidad personal subsidiaria" is the exception rather than the norm, it might be felt that to translate it as "alternative penalty" would be the tail wagging the dog. I don't feel so myself, but if you do, you could put "incarceration or other alternative penalty", as suggested in the answer box.