An art museum in the south of France has discovered that more than half of its collection consists of fakes, in what the local mayor on Sunday described as a “catastrophe” for the region
Japan Today

S An art museum どんな→[in] the south [of] France 美術館
V [has discovered] 発見した
O that

(more than half [of]) its collection コレクションの半分
[consists of]
fakes 偽物

, 補足 
[in what]

S the local mayor ([on] Sunday) 地元の市長
V [described] 述べた
a “catastrophe”
どんな →[for] the region