Proto-Indo-European-speaking Societist South America is so unremittingly cool that I find myself deeply disappointed to know it'll end up with Neo Latin.
As a side note, there's recently been the suggestion that there may have been an even broader language even deeper in the past:
Proto-Eurasian. If it's true, then a group somewhere between the Caucasus and Carpathians was especially successful at taking advantage of the end of the ice age, and one of its descendants - Proto-Indo-European - won out even bigger ten thousand years later.
I'm actually a bit skeptical - especially about Dravidian - but it's a fun idea. There's been an awareness for a long time that Finno-Ugric and Indo-European may once not have been neighboring language families, but neighboring
languages full stop. The vague guess being that they were north-south neighbors and when the PIE's invented the chariot or domesticated the horse or whatever gave them their advantage, the FU's (heh) were the first to learn it off of them. Maybe even the reverse. But since the former were ensconced on the Steppe by most guesses, and the latter were in the mixed terrain north of it, the PIE's were much more successful. At any rate, being at ground zero at the pivotal moment may be why Finno-Ugric was able to survive the expansion at all.
Another thought. OTL, Marxism could frame itself as modern because it was - as an outgrowth of economics - inherently scientific. In practice the science naturally became subordinated to practicality once in government, but it remained as a key part of the legitimacy of the political, ideological, and philosophical systems. Heck, it does to this day! Having math, of all things, to back up an argument, is a powerful foundation even under the assumption that most adherents will never check the math and can't.
Societism doesn't seem to have even that. At most it's more like a philosophy with some inspiration coming (so far) out of Sociology and now Linguistics. I'm not sure off the top of my head how important the lack of "Scientific Societism" will be to the resulting system, but it seems significant. Maybe sympathizers will be attracted to the "dream" or philosophy, where Maxism had relatively more appeal to the analytically minded?
And.... We've already discussed that the pivotal, idealized classes of the ideologies are very different, but it strikes me that the founding ones - well to do academics - are much the same.