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> getdefaultlocale() falls back to LANG and LANGUAGE.
_Py_SetLocaleFromEnv(LC_CTYPE) (e.g. setlocale(LC_CTYPE, "")) gets called at startup, except for the isolated configuration [1].
I think calendar.Locale*Calendar should try the LC_CTYPE locale if LC_TIME is "C", i.e. (None, None). Otherwise, it's introducing new default behavior. For example, with LC_ALL set to "ru_RU.utf8":
3.8:
 >>> locale.getlocale(locale.LC_TIME)
 (None, None)
 >>> locale.getlocale(locale.LC_CTYPE)
 ('ru_RU', 'UTF-8')
 >>> cal = calendar.LocaleTextCalendar()
 >>> cal.formatweekday(0, 15)
 ' Понедельник '
3.11.0a5+:
 >>> locale.getlocale(locale.LC_TIME)
 (None, None)
 >>> locale.getlocale(locale.LC_CTYPE)
 ('ru_RU', 'UTF-8')
 >>> cal = calendar.LocaleTextCalendar()
 >>> cal.formatweekday(0, 15)
 ' Monday '
 >>> locale.setlocale(locale.LC_TIME, '')
 'ru_RU.utf8'
 >>> cal = calendar.LocaleTextCalendar()
 >>> cal.formatweekday(0, 15)
 ' Понедельник '
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[1] https://docs.python.org/3/c-api/init_config.html?#isolated-configuration