need german language for frontend / backend

i have downloaded german language files.
where should i upload the files? i need a quick help.
kindly

sounds great man, thanks a lot
i will check that today.

how can i improe / add new translations?

kindly

in downloaded file there is
2.1x directory

and

master directory

which should i use for 3.x version??

i also translated som files in backend, but nothing changed in frontend?
why? may be also a BUG?