Got it. The parameters used for pluralization were also translated to Spanish: “one” to “uno/una” and “other” to “otros/otras”. Changing them back to English solves the issue. I tested this with my instance by changing the value for too_few_topics_and_posts_notice_MF to the following Spanish translation:
<a href="https://blog.discourse.org/2014/08/building-a-discourse-community/">¡Comencemos la discusión!</a> Hay {currentTopics, plural, one {<strong>#</strong> tema} other {<strong>#</strong> temas}} y {currentPosts, plural, one {<strong>#</strong> publicación} other {<strong>#</strong> publicaciones}}. Los visitantes necesitan más contenido para leer y responder – recomendamos al menos {requiredTopics, plural, one {<strong>#</strong> tema} other {<strong>#</strong> temas}} y {requiredPosts, plural, one {<strong>#</strong> publicación} other {<strong>#</strong> publicaciones}}. Solo los miembros del staff pueden ver este mensaje.
I created a PR to fix this; other texts were compromised as well, not just too_few_topics_and_posts_notice_MF.