Frontend

use translation service to determine placeholder values of date picker (#2827)

* use translation service to determine placeholder values

* added translation of date-picker placeholder values

* added German translations of date-picker placeholder values

* fix lint warning: added missing comma

* fix lint warning: removed trailing spaces

* fixed broken tests

* fixed lint warning

* try to mock TranslateService correctly

* remove TranslateService mock

* removed imports

* fixed lint warnings

* fixed lint warnings

* fixed lint warning

* fixed lint warning

* fixed lint error

* fixed lint errors

* removed TestComponent

* added imports

* removed declarations

* fixed lint error

* fix lint error (invalid sorting)

* finally fixing lint error

* use translation pipe directly in template

* move translation of placeholders into HTML template

* remove TranslateService dependency