The language spec leaves the definition very open-ended:

15.7.4.3 Number.prototype.toLocaleString()

Produces a String value that represents this Number value formatted according to the conventions of the host environment’s current locale. This function is implementation-dependent, and it is permissible, but not encouraged, for it to return the same thing as toString.

Different browsers are allowed to implement it differently, and can implement it differently based on the locale chosen by the user.