Is Application.systemLanguage not giving you enough information, such as user's region (GB or US or somewhere else)?

Now you will be able to differentiate between en_US and en_GB.

That plugin fetches language preferences from settings:
- language (e.g. "en", "es")
- region (e.g. "GB", "US")
- currency code (e.g. GBP, USD)
- currency symbol (e.g. $, £)

Extremely simple API: PreciseLocale.GetCurrencySymbol()

Supports iOS, Android, Windows and MacOSX!
Doesn't require Unity PRO.