It’s a class of people, so in the singular (i.e. “Subhuman”), though the correct way to do it in German would be Untermensch (i.e. first letter capitalized). This is consistent with how racial superiority mindsets dehumanize the “other” so they don’t address them as individuals and instead will describe then as type of people hence singular (the type) not plural (multiple individuals).
If you look up the Nazi version of how they describe those they saw as inferior races, it’s Der Untermensch, not Die untermenschen.
(Cheers for correcting my spelling, by the way).
The inherent racial superiority of one’s own ethnic group is a common theme in far-right ideologies and the use of Untermensch by the Nazis was most definitelly not limited to Jews (ask any Slav or, even better, Roma).
Most definitelly from the outside the de facto behaviour of those in the Israeli government (and in other groups’ such as the military and colonists) seems rooted in the broader feeling of cultural and racial superiority, transcending the “mere” religious kind, so in my opinion the use of Subhuman (or Untermensch, to show the cross-cultural ressonances) rather than merely “Gentile” or “Infidel” seems appropriate.
That said, whilst the rabid racism in Israel does ressonate with that in Nazi Germany, the broader expression of the far-right in the former is most definitelly not the same as in that historical latter and not just because the racial group they treat as superior, and those they treat as inferior, are different: so far Appartheid is the more correct form of describing the expression of racism through the machine of the State and Civil Society in Israel, IMHO, even if the underlying mindset when it comes to beliefs of inherent racial superiority is the same, because its’ expression has been mainly through second class citizen treatment, bullying by the State or with endorsement of the State and frequent closing of eyes by the Authorities to murders across racial lines one way (but not the other) like in Appartheid, not outright extermination like in Nazism.
Hopefully it will not go beyond that, though once this new invasion of Gaza really gets going, it might very well be that we get a genocide of historical proportions, at which point merelly describing it as Appartheid will not be enough.