Updated EmailField error message. This one already available in django translations.

https://github.com/django/django/blob/master/django/conf/locale/ru/LC_MESSAGES/django.po#L343

rest_framework/fields.py

@@ -512,7 +512,7 @@ class EmailField(CharField):
     form_field_class = forms.EmailField
 
     default_error_messages = {
-        'invalid': _('Enter a valid e-mail address.'),
+        'invalid': _('Enter a valid email address.'),
     }
     default_validators = [validators.validate_email]