If your application is supposed to support more than one locale then getting localized format of date/time by changing locale (by means of locale.setlocale()) is discouraged. For explanation why it's a bad idea see Alex Martelli's answer to the the question Using Python locale or equivalent in web applications? (basically locale is global and affects whole application so changing it might change behavior of other parts of application)

You can do it cleanly using Babel package like this:

>>> from datetime import date, datetime, time
>>> from babel.dates import format_date, format_datetime, format_time

>>> d = date(2007, 4, 1)
>>> format_date(d, locale='en')
u'Apr 1, 2007'
>>> format_date(d, locale='de_DE')
u'01.04.2007'

See Date and Time section in Babel's documentation.

The example given by Rob is great, but isn't threadsafe. Here's a version that works with threads:

import locale
import threading

from datetime import datetime
from contextlib import contextmanager


LOCALE_LOCK = threading.Lock()

@contextmanager
def setlocale(name):
    with LOCALE_LOCK:
        saved = locale.setlocale(locale.LC_ALL)
        try:
            yield locale.setlocale(locale.LC_ALL, name)
        finally:
            locale.setlocale(locale.LC_ALL, saved)

# Let's set a non-US locale
locale.setlocale(locale.LC_ALL, 'de_DE.UTF-8')

# Example to write a formatted English date
with setlocale('C'):
    print(datetime.now().strftime('%a, %b')) # e.g. => "Thu, Jun"

# Example to read a formatted English date
with setlocale('C'):
    mydate = datetime.strptime('Thu, Jun', '%a, %b')

It creates a threadsafe context manager using a global lock and allows you to have multiple threads running locale-dependent code by using the LOCALE_LOCK. It also handles exceptions from the yield statement to ensure the original locale is always restored.

locale.setlocale()

You need to setlocale() to ru_RU to parse dates with Russian names for months, weekdays, am/pm, etc.

setlocale() is not thread-safe on most systems.

And then use the correct format codes with datetime.strptime():

from datetime import datetime
import locale

date = '16 август 2021'
locale.setlocale(locale.LC_ALL, 'ru_RU')
d = datetime.strptime(date, '%d %B %Y')
print(repr(d))  # datetime.datetime(2021, 8, 16, 0, 0)
print(d)  # 2021-08-16 00:00:00
locale.setlocale(locale.LC_ALL, '')  # setlocale to the system default once done

And as @deceze mentioned above, you should be parsing the date-string to create a datetime obj, not creating a new string from the string you already have. If you need that, do that as the next step.

# using `d` above
locale.setlocale(locale.LC_ALL, '')  # en for me
print(datetime.strftime(d, '%d %B %Y'))  # 16 August 2021

As of python 3.6 calling astimezone() without a timezone object defaults to the local zone (docs). This means you don't need to import tzlocal and can simply do the following:

#!/usr/bin/env python3

from datetime import datetime, timezone

utc_dt = datetime.now(timezone.utc)

print("Local time {}".format(utc_dt.astimezone().isoformat()))

This script demonstrates a few other ways to show the local timezone using astimezone():

#!/usr/bin/env python3

import pytz
from datetime import datetime, timezone
from tzlocal import get_localzone

utc_dt = datetime.now(timezone.utc)

PST = pytz.timezone("US/Pacific")
EST = pytz.timezone("US/Eastern")
JST = pytz.timezone("Asia/Tokyo")
NZST = pytz.timezone("Pacific/Auckland")

print("Pacific time {}".format(utc_dt.astimezone(PST).isoformat()))
print("Eastern time {}".format(utc_dt.astimezone(EST).isoformat()))
print("UTC time     {}".format(utc_dt.isoformat()))
print("Japan time   {}".format(utc_dt.astimezone(JST).isoformat()))

# Use astimezone() without an argument
print("Local time   {}".format(utc_dt.astimezone().isoformat()))

# Use tzlocal get_localzone
print("Local time   {}".format(utc_dt.astimezone(get_localzone()).isoformat()))

# Explicitly create a pytz timezone object
# Substitute a pytz.timezone object for your timezone
print("Local time   {}".format(utc_dt.astimezone(NZST).isoformat()))

It outputs the following:

$ ./timezones.py 
Pacific time 2019-02-22T17:54:14.957299-08:00
Eastern time 2019-02-22T20:54:14.957299-05:00
UTC time     2019-02-23T01:54:14.957299+00:00
Japan time   2019-02-23T10:54:14.957299+09:00
Local time   2019-02-23T14:54:14.957299+13:00
Local time   2019-02-23T14:54:14.957299+13:00
Local time   2019-02-23T14:54:14.957299+13:00