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When it appears to be rising too steeply, the central banks of major countries cooperate to step in and intervene.
Literal
Rising-too-much [is-ている] [quotative-と] be-seen-[when-とき][topic-は], major-countries [genitive-の] central-banks [subject-が] cooperate-and intervention [to-に] tackle.
Financial-news prose. ~しすぎる ('too much,' 'over-X') attaches to the verb stem of 上昇する, then ~ている marks it as an ongoing/resultative state — 'is in a state of having risen too much.' The frame ~とみられる ('is seen as,' 'is regarded as') is a common journalistic hedge for attributing a perception without naming the perceiver — it's the passive of 見る ('see') functioning as a kind of evidential. ~に当たる ('to be engaged in,' 'to take on the role of doing X') is a slightly elevated alternative to ~する; 介入に当たる is the formal way of saying 'engages in intervention.'