We have a situation for pl_PL (Polish). It seems that for a couple of hours (around 1:30 to 3:30 GMT 12. Sept.) the translation files were messed up. If WP auto-downloaded them during this time, the main translation admin-pl_PL.po (and .mo) would have a few hundred bytes instead of kB. The admin-pl_PL.po was not possible to open. Also, the multisite translation files for 4.0 weren’t downloaded. It was a mess, resulting in half of the admin panel defaulting to English.
After those 2 hours, the translation files seemed to refresh and WP installs that made updates later on were fine. The ones that dl-d the bugged files did not update them, however.
We need someone ( @nacin @ocean90 ) to push an update. Probably just changing the update date in https://api.wordpress.org/translations/core/1.0/?version=4.0 should work.
The maintainer of the pl_PL translation will confirm this below.