feat: re-use the store locale on the client if set on server (nuxt-mo…

…dules#593)

If `vuex.syncLocale` is true, then any locale determined on the
server during SSR should be used as initial value by the client.

This also allows
- Not to determine the language twice when a page is loaded (once on
  the server, then again in the client)
- Other plugins (e.g. user session in a universal app) to alter the
  locale during bootstrap (`await app.i18n.setLocale('xx')`) and make
  sure that that locale is re-used on the client.

src/plugins/main.js

@@ -214,7 +214,9 @@ export default async (context) => {
 
   let locale = app.i18n.defaultLocale || null
 
-  if (app.i18n.differentDomains) {
+  if (vuex && vuex.syncLocale && store && store.state[vuex.moduleName].locale !== '') {
+    locale = store.state[vuex.moduleName].locale
+  } else if (app.i18n.differentDomains) {
     const domainLocale = getLocaleDomain(app.i18n, req)
     locale = domainLocale || locale
   } else if (strategy !== STRATEGIES.NO_PREFIX) {