Using Java's own Locale data for region names

Instead of having to duplicate and manually translate the region names and data with the raw xml files, Java's build-in Locale data could probably be used.

Current we are using a following trick to generate a list of localized country names for the user to select from:

// Code in Kotlin
```
Locale
.getISOCountries() // Fetch list of ISO 3166 country codes
.map { // Create a new Locale in users default language, but set country to X
it to Locale("", it).displayCountry // then extract the localized name of the country.
}
.toMap() // Turn into Map with ISO 3166 key and Localised country name as value
```

The issue mentions the raw XML files and the Kotlin Locale.getISOCountries().displayCountry flow, but no repository files or tests. Locate the current region-name data and generation entry point, then compare them with Java Locale behavior. Done means the agreed data source is selected and the country-name generation is updated with verification for the existing use case.

索引モデルが issue の本文から書いたものです。