As a highly mobile organization operating in more than 125 countries, UNHCR is deeply committed to the principle of multilingualism and requires its international professional staff, upon recruitment, to demonstrate competence in at least two official UN languages. Therefore, candidates aspiring to join the international professional category must be able to demonstrate, at a minimum, proficiency at an advanced level (C1 or above) in either one of the two working languages of UNHCR (English and French) as well as knowledge of a second official UN language at an intermediate level (B2 or above). If you have proficiency in French at the C 1 level then, you will have to demonstrate proficiency at B2 level in English.
A certificate is not required in respect of a UN language that is the mother tongue of the candidate. Otherwise, certification from an institute recognized by UNHCR is obligatory.
The list of certificates recognized by UNHCR to prove language proficiency at C1 and B2 levels can be found here.