The honest answer
English signage at Japanese stadiums is improving but still limited. Major stadiums (Tokyo Dome, PayPay Dome, ES CON Field) have basic English signs for exits, restrooms, and section numbers. Smaller stadiums may have Japanese only. However, the visual nature of sports means you can enjoy a game with zero Japanese.
What's usually in English
Exit signs, restroom signs, section/gate numbers, scoreboard player names (in romaji), emergency information. Some stadiums have English stadium maps available at information counters — just ask.
What's usually Japanese only
Food stall menus, announcements over the PA system, event information, ticket categories, team apps. This is where your phone becomes essential — Google Translate's camera mode can read menus in real-time.
Essential Japanese for game day
入口 (iriguchi) = entrance. 出口 (deguchi) = exit. トイレ (toire) = toilet. ビール (biiru) = beer. 指定席 (shiteiseki) = reserved seat. 自由席 (jiyuuseki) = unreserved seat. 外野 (gaiya) = outfield. 内野 (naiya) = infield.
Helpful apps
Google Translate (camera mode for signs and menus). Google Maps (stadium navigation and transit). NPB official app (scores and stats — partially in English). Team apps (Japanese only but useful for QR ticket display).
Staff helpfulness
Japanese stadium staff are exceptionally helpful. Even without English, they'll use gestures, draw maps, and go out of their way to assist you. Showing your ticket or pointing at your phone screen communicates most needs. Some stadiums near tourist areas (Tokyo, Osaka, Kyoto) have staff with basic English.
Pro tip
Screenshot important information (seat number, gate, directions) on your phone before you arrive. This makes communication much easier. Write your hotel address in Japanese on a card in case you need help getting back after the game.