What do you look like when you speak Ancient Greek (Koine) in Greece today?

How soon my fellow respondents forget Katharevousa. Just as well they do, too.

Katharevousa (Puristic Greek), the project of purifying Greek of the last 2000 years of linguistic evolution, was a motley, incoherent, and rarely lovely thing. Some of its grammar was Attic, a lot more of it was Koine, and by accident it ended up most similar to Mediaeval Greek (purists were winding back the clock as much as  was feasible, and not much was feasible).

Nonetheless, a Frank such as OP, trying to speak Koine in Greece with Modern Greek pronunciation, would be taken for a valiant attempt to speak a somewhat over the top Katharevousa. And given how much  of a moving target Katharevousa was, they would probably get away with it too.

So 130 years ago, OP would be fêted and complimented for their excellent Greek by the elite—who would mutter to each other “Look you, the Frank speaks better Hellenic than us!” The common herd would bow and walk away, with their prejudice confirmed that the Franks speak the same gibberish as the high and mighty.

80 years ago, OP would be complimented for their  excellent Greek by the establishment. Not fêted though. The more atticising versions of Katharevousa had already died out, so even the establishment would have started thinking OP odd. Since noone used Katharevousa in literature after 1900 (Cavafy doesn’t count), the intelligentsia would look askance at OP, and probably play practical jokes on them. The communists would beat OP up in an alley, convinced they were a British spy.

30 years ago, OP would have been greeted with gales of laughter. Katharevousa died 40 years ago, and it died through ridicule, as much as it did through guilt by association with the Colonels’ regime. Any survivals of Katharevousa by 1980 were jocular.