When asked this change: https://bugzilla.wikimedia.org/show_bug.cgi?id=34977 made a mistake.
4-digit numbers should not be separated by a NON-BREAKING SPACE ( ). According to Spanish standards: http://lema.rae.es/dpd/?key=n%C3%BAmeros#2 (Real Academia Española)
Cite: "Los números de cuatro cifras se escriben sin espacios de separación: 2458 (no 2 458)"
i.e.: see https://es.wikipedia.org/wiki/WP:N%C3%9AMERO
Version: 1.22.0
Severity: enhancement
Thanks for taking the time to report this!
Related patchset: https://git.wikimedia.org/commitdiff/mediawiki%2Fcore.git/0f876a3c239252233b8466ae89e299883d3b9ed1
You're forgetting the sentence before that: «Al escribir números de más de cuatro cifras, se agruparán estas de tres en tres, empezando por la derecha, y separando los grupos por espacios en blanco: 8 327 451 (y no por puntos o comas, como, dependiendo de las zonas, se hacía hasta ahora: x8.327.451; x8,327,451)».
The non-breaking space is the correct separator; as in Italian (and perhaps other languages), it's usually not added for 4-digits numbers (i.e. for 1-digit thousands group) because it's overkill; updated summary. No idea about Spanish but in Italian this is a secondary rule, of lesser importance.
I don't know what it would take to implement such an exception to the formatting rule.
I think this is implemented for Polish, so it should be easy enough to copy that code to Spanish.
Change 76306 had a related patch set uploaded by Nemo bis:
4-digits numbers in Spanish should not have a group separator
Change 76306 merged by jenkins-bot:
4-digits numbers in Spanish should not have a group separator