The Debian package should register icu4j-localespi as Java extension

XWiki embeds ICU4J because it's a dependency of Jython but in practice it's not really used by anything else than Jython because it's not loaded by Java as alternate Locales. This can only be done through java ext libraries which cannot be provided by a WAR unfortunately. It's a pity since XWiki have several translations for locales which are not supported by Java (fa, si, km).

See https://sites.google.com/site/icuprojectuserguide/icu4j-locale-service-provider

But that does not prevent us from injecting ICU4J in setups on which we have more control (like the XWiki Jetty package, the Debian packages and the Docker image).

Note that since Java 9 all locales supported by CLDR (the source of ICU4J) are embedded in Java making this issue a lot less important (could still be interesting to have really the latest locales update depending on which version of Java runtime you are using).