Chinese options currently only include Hong Kong, Macau, and Singapore. Chinese in Taiwan often has specific translations for them, but there is no option for Taiwan.
English offers 27 different countries/dialects (including two made up ones: upside-down and Pirate)
Spanish offers 22 different countries/dialects.
French offers 6.
etc.
A custom language can be added only by the project owner, that is, it can be done only from the main account
If the button is not available for you, then you are not the owner of the project and therefore you need to contact the owner of the project and ask to add a custom language