Should my Android app default to simplified or traditional Chinese?

Android by default assumes zh to be Traditional by default, but Unicode (of which sets the standard for locale usage) uses Simplified for zh.

If your app's minimum version is Android 7.0 (API 24), then you can utilise the new locale resolution. By defining the folders as values-b+zh+Hans (Standard) and values-b+zh+Hant (Traditional), the system will automatically use the correct locale depending on which region was selected. You can still use values-zh-rTW and the like, if you need regional differences between the regions using the same variant.

Traditional Chinese characters (Standard characters) are referred to by several different names within the Chinese-speaking world. The government of Taiwan officially calls traditional Chinese characters standard characters or orthodox characters (traditional Chinese; simplified Chinese.

People in China, Malaysia and Singapore mainly use the simplified system, Traditional Chinese characters are currently used in Taiwan (Republic of China), Hong Kong and Macau.

I think that you should not omit that values-zh , put it as simplified Chinese or allow user to switch the language in your apps.