Simplified getting __lang__ from provider

Looking into how `python -m faker > docs.txt` works to generate documentation, noticed that the approach to determine the `__lang__` of a provider was a bit strange. Therefore this simplification to help maintenance... 

Background: the following works interactively:

```
>>> from faker import Faker
>>> fake = Faker('nl_NL')
>>> for p in fake.get_providers():
...     print p.__lang__, '\t', p.__provider__
... 
None 	user_agent
nl_NL 	ssn
nl_NL 	phone_number
None 	python
None 	profile
nl_NL 	person
None 	misc
None 	lorem
None 	job
None 	internet
None 	file
None 	date_time
None 	credit_card
en_US 	company
None 	color
nl_NL 	address
None 	base
>>>
```

Note also how the `company` provider is "en_US" (the default) as that hasn't been localized (yet) for "nl_NL".