GNU date won't parse it's own output

[dronus]

On Ubuntu 18.04, using german locale, GNU date (v 8.28) doesn't accept it's own output format using date -d. This is awkward, as date -d is meant to parse all "natural" formats, and of course the output of date will often be stored and need to be read again.

What have I done?
Run the test case
    date -d "`date`"
on up-to-date Ubuntu 18.04 with de_DE.UTF-8 locale.

What happens?
date complains that the input has invalid date format.

What I expect to happen?
GNU 'date -d' should be able to read the date text created by 'date' without further options.