What determines the language splits installed with apps?

I have an Android 10 device that I recently updated the (custom) ROM on, and now (for apps that have splits that include language splits - for example, config.en.apk) when installing or updating an app, I get not only English, but Spanish (config.es.apk), as well. In System / Languages, I have only English (United States) selected. Installing the same apps on other devices installs English only (the aforementioned config.en.apk). What setting or configuration option forces apps to add support for Spanish?