"What the tongue is, the man is". Explain.

In this context, the "tongue" refers to native tongue or language. In this way, Madame Defarge classifies the spy as English.

“Going?” echoed madame. “She was pretty enough to have been married long ago. You English are cold, it seems to me.”

“Oh! You know I am English.”

“I perceive your tongue is,” returned madame; “and what the tongue is, I suppose the man is.”