So, just as you would surely forgive me if I really were a foreigner and spoke in the dialect and manner I had been brought up in,

Foreigners could not address an Athenian court in their own right, so the comparison is a hypothesis: if I were one. Phōnē is "voice," and here the voice a person grows up with, their accent and dialect. Ξένος, xenos, is the word for the stranger, the guest and the foreigner at once. It is the root of "xenophobia," and the guest-friendship the Greeks considered sacred. The sentence does not end here. It runs on into the request that follows, and the Greek holds the reader in suspense across the page.