The point is that EECMS uses two folders of languages!
/system/expressionengine/language/ and the other is
/system/codeigniter/system/language. Both language packs are completely different.
The dates and the form validation messages are used from the one in the Codeigniter folder.
You can find CodeIgniter Language Packs on github, but I don't know if they match the custom version used on EECMS.
My suggestion is to duplicate the contents of the English folder and translate just the parts you need.
Keep the same name on both folders and they will be activated together.
Yeap! This is annoying. I already filled a bug report about this, but looks like, despite of what the bug report says, it is still relevant:
This bug report is over one year old and may not be relevant.