AION 2 Global server regions currently confirmed: North America, South America, Europe and Japan. See Oceania/SEA status and what server details are still unannounced.
Japan, Europe or another available region may end up being the practical choice depending on routing, but a generic geographic guess is not a ping test. Wait for the live client/server list, then choose based on friends/Legion first and measured latency second. This guide will not publish fake “JP = X ms” estimates before the routes exist.
The Global client announces eight supported languages, while the server-location list currently names four regions. Do not assume a language automatically has its own server region.
Official Steam pre-release FAQ — server locations