Do you need a plug adapter?

Japan

Japan runs on 100 volts — the lowest mains voltage anywhere. Most chargers cope; anything with a heating element or a motor often does not.

China

Often not. Sockets here accept flat two-pin, round two-pin and angled three-pin plugs, so most travellers' plugs fit as they are.

Thailand

Your plug probably fits — Thai sockets take flat American pins and round European ones alike. That is the trap, because the supply is 220 volts: fitting and working are two different questions.

Vietnam

220 volts, and most sockets take two round pins. European plugs go in as they are; American and Japanese ones need an adapter — which the tourism board says any electrical shop will sell you.

South Korea

Yes, unless you already carry a European one. Korea runs 220V at 60Hz on round two-pin sockets.

Taiwan

Taiwan runs on 110V with flat two-pin sockets — the American pattern. European and Australian plugs need an adapter; some devices need more.