google.auth.exceptions.DefaultCredentialsError:

Your need to set an environment variable for GOOGLE_APPLICATION_CREDENTIALS

You can add this in your code by adding in the following lines:

credential_path = "D:\Summer Projects\Translate\social media analysis-2a59d94ba22d.json"
os.environ['GOOGLE_APPLICATION_CREDENTIALS'] = credential_path

Complete solution:

# Imports the Google Cloud client library

import os    
from google.cloud import translate

credential_path = "D:\Summer Projects\Translate\social media analysis-2a59d94ba22d.json"
os.environ['GOOGLE_APPLICATION_CREDENTIALS'] = credential_path

# Instantiates a client
translate_client = translate.Client()

# The text to translate
text = u'Hello, world!'
# The target language
target = 'ru'

# Translates some text into Russian
translation = translate_client.translate(
    text,
    target_language=target)

print(u'Text: {}'.format(text))
print(u'Translation: {}'.format(translation['translatedText']))