In 30 seconds
Use simplified characters for mainland China and Singapore, traditional characters for Taiwan, Hong Kong and Macau. Written script and spoken language are separate decisions: voice-over also needs the right spoken variety.
Where each one is used
Simplified Chinese is the standard written form in mainland China and in Singapore. It uses characters with fewer strokes, after a reform of the written language in the twentieth century.
Traditional Chinese is the standard written form in Taiwan, Hong Kong and Macau, and keeps the older, more complex character forms.
The word “Chinese” also covers spoken varieties. Mandarin is the common spoken language of mainland China and Taiwan. Cantonese is the everyday spoken language of Hong Kong and of much of Guangdong province. Written and spoken are separate decisions, and a project often needs to settle both.
It is more than the characters
The same idea is often written with different words in different markets. The word for software, for example, is 软件 in mainland China and 軟體 in Taiwan. A reviewer who knows the destination catches this; a character converter does not. Punctuation and number formats also differ in places, and designers need to check line breaks and fonts for each script.
How to decide, by deliverable
Website or app text. Pick the script of the market you are launching in. Launching in several markets means more than one version, and each should be reviewed by someone from that market.
Subtitles. Follow the viewer, not the speaker. A film with Mandarin audio for Taiwan gets traditional characters; the same film for mainland China gets simplified.
Voice-over. Choose the spoken variety first: Mandarin for the mainland and Taiwan, Cantonese for Hong Kong. Then pick a voice with the accent your listener expects. The studio has voices in all of these; the Chinese language page lists the varieties and links to the brand that casts them.
On-screen text in video. Captions and graphics must match the script of the subtitles. Decide before the edit, because redoing titles later means going back into the project.
What to tell your producer
Target markets, in order of importance. Where the video or text will run. Whether the voice should be Mandarin or Cantonese. Names, product terms and slogans that must stay as they are. A native reviewer on your side, if you have one.
If the market list is unclear, begin with the largest market and add the others once the first version has been checked by a reader from there.
Quick answers
Can I convert simplified text into traditional automatically?
Characters can be converted by software, but vocabulary, terms and phrasing differ by market. A converted text usually still needs a native reviewer who knows the destination.
Do Taiwan and Hong Kong use the same traditional Chinese?
They use the same script, but wording and style differ, and spoken Hong Kong is usually Cantonese. Treat them as separate markets when the text is customer-facing.
Which one should I pick if I can only make one version?
The one used where most of your customers or viewers are. If you do not know yet, say so to the producer: it is a market question, not a language one.