[Ring] Change "da_DK" to "da" for danish translation

[scootergrisen]

I suggest changing from using "da_DK" to "da" for danish translation.

On Transifex:
Change team from "da_DK" to "da".

In da_DK.po file change:
"Language-Team: Danish (Denmark) 
(http://www.transifex.com/savoirfairelinux/ring/language/da_DK/)\n"
To:
"Language-Team: Danish 
(http://www.transifex.com/savoirfairelinux/ring/language/da/)\n"

And change:
"Language: da_DK\n"
To:
"Language: da\n"

Change file name "da_DK.po" to "da.po".

That way we are more consistently using "da" for software translated to 
danish instead of some software using "da_DK" and some use "da".
I prefer "da" because its shorter and have so far never seen more then 
one kind of danish translation used in software.