。
Swiss chocolate melts beautifully in your mouth.
Literal
Switzerland [gen-の] chocolate [topic-は] mouth-melt [subj-が] is-good.
口溶け 'melting in the mouth' is a compact compound noun from 口 'mouth' + 溶け (from 溶ける 'to melt') — foodie vocabulary used especially in chocolate, ice cream, and butter reviews. 口溶けがよい '(has) good mouth-melt' is the stock collocation. The whole sentence is a nominal evaluation of a food quality, structured as [topic]は[quality]がいい — a very productive frame.