The main signal is the locale setting, which can have a list of languages in it. For example, you could have Finnish as your primary language, with Hong Kong Ch

The main signal is the locale setting, which can have a list of languages in it. For example, you could have Finnish as your primary language, with Hong Kong Chinese second. Then CJK will get rendered as zh-Hant-HK. Apps can set locale explicitly to override that, but almost none do. Probably the motivating example is a translation app, which can then render both, say, Chinese and Japanese correctly. Input methods are technically a separate setting from your locale list, which is maybe confusing but that's the way it is.