Add support for private subtags in translation's to_locale().

[Ronnie van den Crommenacker]

In Dutch we make a difference between formal and informal communications.
We're starting to implement this by using a private subtag nl-nl-x-informal
​https://www.w3.org/International/articles/language-tags/#extension

Because of the sr-latn exception in the to_locale function the private subtag is formatted wrong.

>>> from django.utils.translation.trans_real import to_locale
>>> to_locale('nl-nl-x-informal')
nl_Nl-x-informal

While the country code should be with a capital L:

nl_NL-x-informal

[Brent Hand]

I have added the check to make sure that subtags are formatted properly. Here is the link to the PR ​https://github.com/django/django/pull/8995

[Tim Graham <timograham@…>]

In 0c4ac12:

Fixed #28546 -- Fixed translation's to_locale() with langauge subtags.

Thanks Brent Hand for the initial patch.