See hed. The nuance of “grunt” is not exactly correct in that short definition. Think of “grunting” and the image is clearer.
Which reminds me, I don’t know what the equivalent in the other US services is. Even though every recruit can wind up doing slogging heavy work and be put-upon, I associate the feeling of the word with Armies only.
In the UK they say squaddie. The pioneer corps were known as chunky’s
In Spain recent recruits are pelados (“shorn” for evident reasons, and usually pronounced without the d), quintos (because when we had a draft, any group of draftees would be born in the same year except for those with deferrals - people born in the same year are quintos and the group is your quinta)… I’ll have to ask my brother who actually did his service what other terms he knows, but he may apply a “not for ladylike ears” filter (as if I couldn’t cuss him to the ground). A corporal is called a sardineta (tiny sardine), a term which also refers to each of the stripes.
During WW2, the top officers of the Japanese army referred to their foot soldiers as “issen gorin.”
In those days, “Issen gorin” was the price, in Japanese money, of a postcard. The implication was that an ordinary Japanese soldier was expendable and easily replaced. If one Japanese boyr died, it cost almost nothing to send out a postcard conscripting another.
In WWI France they were called “poilu,” meaning “hairy one.” But don’t Google it at work: tends to yield French porn.
I explained this upside-down, comes from typing and cooking at the same time.
The draft was 20% at one point, so 1/5 (un quinto) of the males born in that year got drafted, which leads both to recruits being called quintos and to people born in the same year to get the same name, with the group of people born in a given year being that year’s quinta.
soldado raso, recluta (recruit) or, when compulsory military service (“colimba”) existed: colimbas.
“Pioneer corps?”
Also, this is infantry, right? And, derivation of “chunky?”
(That’s the fun of this.)
Yeah, but soldado raso is the actual name of the grade, so not slang.
For the German army soldier in WWII, this would have been: Landser
True.
What the American army calls Combat Engineers or Sappers.
The OP’s premise is flawed. A grunt is not the lowest ranking soldier. A grunt is an infantry soldier of any rank. Or as we called them while I was in tanks, crunchies.
Or: Abnutzungskriegkanonenfuttersoldaten. 
(j/k)
In the Navy or Coast Guard, it’s “swabbie”. Swab the deck.
Well, in Azeroth, the lowest units in the Horde are Grunzer (German), Bruto (Spanish), and I think just Grunt in French; will need to search more to find more. :dubious:
The pioneer Corps were the ones that went in first to do the digging of latrines, ditches etc. I’m not 100% sure, but the term chunky was something to do with “the men with the pineapple balls” Don’t ask me why. I was in the UK Military Police in the late fifties and we had derogatory terms for most of the other regiments. For example, the Intelligence corps were known as "the clever c***s.
“Bidasse” in French comes to mind, but it was used for conscripts, and I’m not sure it can be properly used for professional soldiers (never wondered before today).
Pakistan, it’s jawaan, literally “youngster”.
Ditto India, but in India at least, it’s not really ‘slang’ per se. You’ll see it used in all sorts of formal ways and contexts.
ETA: What I mean is, I wouldn’t view jawaan to be strictly equivalent to ‘grunt’ or the other examples given here. Unfortunately I’m not conversant enough with military slang to know if there even is an alternative that fits better.