Превод - Холандски-Английски - jammer dat je er niet was,hard gewerkt?

My suggestion :
Too bad you weren't there, have you been working hard?

i think it should be

i'm sorry you were not there, have you been working hard?

in this way its less formal, and the original text seems not that formal to me.

'Too bad...' sounds much better than 'Regrettably...'

Agree, "too bad" or "I'm sorry" should be used instead of "Regrettably".

Ik zou, ipv "regrettably" kiezen voor: "too bad"(=meer spreektaal) ligt natuurlijk aan de verhouding tussen jou en de ontvanger van deze tekst.