When he gets angry, he often uses foul language.

Literal

Belly [object-を] get-angry-[when-と] he [topic-は] often foully curses.

口汚くののしる means 'to curse foully' — 口汚い (foul-mouthed) in adverbial form + ののしる (to abuse verbally). よく here means 'often.' The sentence describes a habitual reaction.