Which internationalization technique would work best with an MGWT, GWT-phonegap app that is eventually to be converted using cordova to a native app for dynamic locale setting?
GWT provides static internationalization methodology (using Constants and Messages), but I do not think it is possible to set the locale dynamically with these.
Does MGWT provide any internationalization options?
You can use GWT static i18n. It produces a highly compact permutation for each language. Then you can package each permutation into a different Cordova (Phonegap) app - one per language. The advantage of this approach is that the app size will be much smaller as only one language is included. I would recommend this approach if your app has a lot of Strings and texts.
An alternative solution is to use Cordova's globalization plugin. Note all the quirks, however, described in the documentation.