Compared across countries

Do you need a plug adapter?

For a trip that crosses borders — the same question, answered country by country.

The pattern

Two separate questions, and the region answers them independently. Voltage splits the map — 100V in Japan, 110V in Taiwan, 220V in the other four — while the socket shape does not follow that split, because China and Thailand accept flat and round pins alike.

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.

Sort the practical stuff before you fly.