I am stuck on a similar decision too. Managed to get into HEC but somehow seem to favour St. Gallen over that. Any idea how the placements are like for non german speaking Non EU students?
Well, the St. Gallen program was "internationalized" only a few years ago and therefore I doubt you'll find many graduates with such a background. I'd contact the University and ask them if you could be put in touch with a recent alumni from your country/region. Then ask him/her.
I did my undergrad degree in St. Gallen and can't really tell. There's certainly a very international job market in Zurich with a lot of companies that depend on the "global talent pool", but from my experience they are very open to help out experienced applicants with visa/working permits but on the graduate level it's another story.
If you do it, you should have a backup plan in mind in your home country. But if you're interested in coming to the German speaking part of Europe later on, the St. Gallen degree will definitely help. And since the St. Gallen degree is a bargain compared other top programs, there's not too much to lose.
Can't really say much about HEC Paris, I think it has exactly the same position as St. Gallen in its linguistic region for Europeans, but I have absolutely no idea how placements are if you're not from the EU. But speaking French there will be at least as important as speaking German in St. Gallen.
SSE: Well, for London recruiting it's probably the same as St. Gallen and Bocconi (possible, but just part of the big group of schools that are good but not LSE/Oxbridge). But if you're more thinking of the local markets (Zurich/Geneva/Frankfurt) vs Stockholm, I think you'll be better off in St. Gallen, simply because it places great in all of German speaking Europe which is much bigger than SSE's home market.