The locale names in Java8 replace the ones in Java 7, e.g. en_GB became en-GB

As of MAGNOLIA-7897 Magnolia can seamlessly support both formats in configuration, e.g. property fallbackLocale at https://documentation.magnolia-cms.com/display/DOCS62/Language+configuration#Languageconfiguration-3.Definelocalesinsite.

Probably worth mentioning it in our docu, besides the fact that the new Java8 locale names are preferred anyway.