I believe the key question is whether we are talking about an essential skill. Let me give you an example.
Imagine the position of a chair of a tribunal where one of the hiring criteria is that that individual be a lawyer. I find it hard to see how one could appoint an interim chair who is not a lawyer. I find it hard to imagine how anyone could say this is not serious because that individual is only an interim chair. If it is essential for the chair to be a lawyer, then his or her replacement must be one as well. Similarly, if the role of agent of Parliament is such that it is essential that that person understand and communicate with parliamentarians and the public in both official languages, then that must apply to the person who replaces that individual. That was the case, for example, when Sheila Fraser left her position as Auditor General and was replaced.
The people who work for agents of Parliament are generally public servants. They occupy positions in which bilingualism is essential. Consequently, it is highly unusual in real life to appoint on an interim basis a person who does not have that ability.