I just wanted to note that the description and title may cause a wrong interpretation of what should happen:

If you first set LC_ALL and then one of the other categories such as LC_NUMERIC, locale C functions will still use the LC_ALL setting for everything. LC_NUMERIC does not override the LC_ALL setting.

I tested this on OpenSUSE and get the same wrong results. Apparently, locale.localeconv() does not respect the above order. That's a bug.

I'm not sure whether the OP's quoted behavior is a bug, though, since if the locale encoding is not UTF-8, you cannot really expect using UTF-8 numeric separators to output correctly.