App does not use system language preference when expected

[achou11]

Describe the bug

The flow in which we should be calculating the language to use in the app is generally as follows:

1. If you explicitly selected a language in the app using the language settings, use that
2. Use the the language preference based on the system preferences
3. Use English

Assuming (1) is has not been done, switching the system language to a choice that the app does support does not update the the language in the app. This is the case the app is already opened and even after restarting the app.

To Reproduce
Steps to reproduce the behavior:

1. Do a fresh install
2. Open your Settings app (do not close CoMapeo)
3. Go to System -> Languages
4. Change the primary (or preferred, depending on Android version) language to something other than the current choice that is supported by the app
5. Go back to the app. You'll notice that the app does not update to make use of the newly selected system language
6. Restart the app. Upon re-opening, you'll notice that the app does not make use of the newly selected system language.

Expected behavior

In the case that the user has NEVER explicitly selected a language within the app's settings UI, the chosen language based on the user's system preferences should be reflected in the app.

Screenshots

system-language-issue.mp4

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: Pixel 6
• OS: Android 15
• Version: 1.2.0

Additional context

I think I already identified the issue in the code. It's related to our usage of persisted state for initializing and determining the active language that the app uses

https://github.com/digidem/comapeo-mobile/blob/309c374022d1bc46bfade1376761fd80113bb351/src/frontend/hooks/persistedState/usePersistedLocale.ts

[achou11]

Part of the issue here is that we don't have a way of understanding why the active language is being used. For example, I see that English is my selected language, but I have no idea if that's because my system preferences are being used, or if it's because at some point, I pressed the English option and our app persisted that in storage.

Most apps I use have an option in their language selection UIs along the lines of Follow system preferences and usually that's the default. Would be great to have something similar for ours or update the UI to communicate that somehow.