Here "tongue" refers to what the officials say. Alternate translation: "because they insult me" or "because they curse me" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The abstract noun "mockery" can be stated as an action. Alternate translation: "This is why people in Egypt will mock and laugh at Israel" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])