ECMA 262 specifies that the function is implementation dependent and takes no arguments.
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.
NOTE The first parameter to this function is likely to be used in a
future version of this standard; it is recommended that
implementations do not use this parameter position for anything else.
It is also in ECMA internationalization API specification (which for Number.prototype.toLocaleString
supersedes ECMA 262 but accepts 2 arguments)
This definition supersedes the definition provided in ES5, 15.7.4.3.
When the toLocaleString method is called with optional arguments
locales and options, the following steps are taken:
Let x be this Number value (as defined in ES5, 15.7.4). If locales is
not provided, then let locales be undefined. If options is not
provided, then let options be undefined. Let numberFormat be the
result of creating a new object as if by the expression new
Intl.NumberFormat(locales, options) where Intl.NumberFormat is the
standard built-in constructor defined in 11.1.3. Return the result of
calling the FormatNumber abstract operation (defined in 11.3.2) with
arguments numberFormat and x. The value of the length property of the
toLocaleString method is 0.
Besides, mdn specifies that Safari has no support for it.
As for a viable solution see this answer on SO