Ok take a look at com.google.gwt.i18n.I18N.gwt.xml
Seems like there are 5 ways to provide the locale:
1.) using a query param named "locale". To use this method you can let your web server send a redirect from app.example.com
<locale> after determining the locale on your web server if possible or you do the redirect from within your app, e.g. in your onModuleLoad() you use Window.Location.assign(<current url> + <locale query param>). You can change the name of the query param by setting a different value to "locale.queryparam".
2.) using a cookie. To use this you have to define the cookie name by setting "locale.cookie" to any value as in I18N.gwt.xml no default cookie name is defined.
3.) using meta tags. As already described you can include a gwt:property meta tag in a dynamic host page.
4.) using the user agent information. To use this you have to activate it by setting "locale.useragent" to "Y" as its disabled by default in I18N.gwt.xml.
GWT's default search order is "query param, cookie, meta, useragent" but cookie and useragent will be skipped if you don't configure/activate them. You could also modify the search order by setting "locale.searchorder" in your gwt.xml.
Now choose one solution ...