Problem/Motivation
Comment for \Drupal\Core\Datetime\DateHelper::dayOfWeekName() says it returns translated string and it really make sense to return translation here. But in fact now it returns untranslated English name of the day. This change was brought by the
#3281557: DateTime::__construct(): Passing null to parameter #1 ($datetime) of type string is deprecated . I didn't find the related discussion, suppose it's changed by mistake.
The change to use an untranslated string was added in #3281557-54: DateTime::__construct(): Passing null to parameter #1 ($datetime) of type string is deprecated. No explanation is given and it is not discussed in later comments.
Proposed resolution
Return the original behavior - return translation from the method.
Remaining tasks
User interface changes
No.
API changes
No.
Data model changes
Just return state to previous.