The translator found it impossible to explain what he was saying.

Literal

Translator [topic-は], he [subject-が] saying thing [object-を] explain [nominalizer-の] [topic-は] impossible [copula-だと] understood.

彼の言っていること ('what he is saying') uses ている for ongoing speech and こと to nominalize it. 不可能だとわかった ('found it to be impossible') reports a realization through と + わかる.