Locale states that

"Well-formed {@code variant} values have the form {@code SUBTAG (('_'|'-') SUBTAG)*} ... BCP 47 only uses hyphen ('-') as a delimiter, {@code Locale} is more lenient."

However, there is a wide range of behavior which makes the lack of detail confusing.

`Locale.of` can take multiple variants separated by underscore.
`Locale.forLanguageTag` allows multiple variants separated by hyphen.
`Locale.Builder.setLanguageTag` allows variants only separated by hyphen.
`Locale.Builder.setVariant` allows multiple variants separated by either underscore or hyphen.