Agree with the comments, that we need something neutral.
Space would do it, and while this is used by specific countries, others who use comma/dot/other separator should be able to adapt to spaces easily.
This should be fine wherever there is never going to be a decimal point. Otherwise the issue is still present where different folks will be familiar with different characters.
What about using a letter for numbers from 10,000 up, as a simple way to visually show million (M) or thousand (K)?
This should be enough to make it more clear at a glance.
After thinking about it.
Anything under ten thousand is easily recognisable without requiring anything further (
5733), so can remove those from the equation completely.
5 chars is fairly easy to recognise:
If we use
K for "hundred thousands" only, and
M for "millions" only, they both become instantly recognisable.
K = hundred thousands
M = millions
Even though the average Subitizing is only about 4, with the familiarity (3) introduces, and considering (1) is negated entirely, this combined "familiarity" makes recognising ten thousands (10000 to 99999) also naturally recognisable, because they are not "the others".
So all number ranges would have a quick to recognise attribute.
hmm... maybe I just need sleep...
Either way, M and K seems legit to me.