Language fixes (#5384)

* Add some comments

* Decouple language settings

* Normalize on read/write

* Refactor

* Support device locale on app startup

* Cleanup, port to web

* Clean up comments

* Comment

* Try not to mutate

* Protect util handling, update test

* Dedupe array values

package.json

@@ -110,6 +110,7 @@
     "await-lock": "^2.2.2",
     "babel-plugin-transform-remove-console": "^6.9.4",
     "base64-js": "^1.5.1",
+    "bcp-47": "^2.1.0",
     "bcp-47-match": "^2.0.3",
     "date-fns": "^2.30.0",
     "deprecated-react-native-prop-types": "^5.0.0",