Improve automatic locale mapping

Problem/Motivation

Locale mapper is missing multiple countries and should take purchase country and interface language into account when resolving the locale.

Remaining tasks

Map purchase country and interface language to a locale. For example:

1. Finnish interface language and finland as purchase country should resolve to fi-Fi
2. English interface language and finland purchase country should resolve to en-FI
3. If no match is found, fallback to purchase country's default locale, in this case fi-Fi
4. If no match is found for country, fallback to en-US

See https://docs.klarna.com/klarna-payments/in-depth-knowledge/puchase-count... for available locales.