“en-US” is an IETF language tag. While Java’a Locale class was clearly based on IETF language tags, it uses underscores in place of hyphens when separating language codes from country codes (and also variants), so calling toString() on the equivalent Locale will give you en_US.
As of Java 7 you can use Locale.forLanguageTag(String) and toLanguageTag() to convert between language tags and Locale objects.
When converting strings to Locale objects it’s a good idea to normalize by splitting components on hyphens and underscores, lowercasing the first component (the language code) and upper-casing the second component (the country code).