GWT has an amazing mechanism to localise Applications. It uses CLDR and ICU4J as source data to generate it's own java stuff so as it can be used in both: client and server sides. Server side is a work on progress, and is partially functional, probably the entire implementation will be released with GWT-3.0.0 in during 2015-Q1.
The thing is that GWT internationalisation stuff has not been updated for more than 2 years since its main maintainer left Google, remaining with the CLDR release-21 while latest is v25.
I became a GWT maintainer after I joined Vaadin some months ago, and one of the parts I maintain is internationalisation.
The good news is, that some time ago, we have been working on updating this stuff, adapting migration scripts and fixing issues.
Finally and thanks to the hard work of gwt maintainers and googlers, we have updated GWT with CLDR-v25, you can take a look to the big patch at the gwt git repository. I hope we can maintain GWT updated to last CLDR changes from now.
You can use the new stuff using latest 2.7.0-SNAPSHOT, until a new version of GWT is released at the end of September.