Example of the bug:

import locale
# LC_CTYPE: latin1 encoding
locale.setlocale(locale.LC_ALL, "en_GB")
# LC_MONETARY: utf8 encoding
locale.setlocale(locale.LC_MONETARY, "ar_SA.UTF-8")
lc = locale.localeconv()
for attr in (
    "mon_grouping",
    "int_curr_symbol",
    "currency_symbol",
    "mon_decimal_point",
    "mon_thousands_sep",
):
    print(f"{attr}: {lc[attr]!a}")

Python 3.7 output:

mon_grouping: []
int_curr_symbol: 'SAR '
currency_symbol: '\xd8\xb1.\xd8\xb3'
mon_decimal_point: '.'
mon_thousands_sep: ''

Expected output:

mon_grouping: []
int_curr_symbol: 'SAR '
currency_symbol: '\u0631.\u0633'
mon_decimal_point: '.'
mon_thousands_sep: ''

Tested on Fedora 29.