Change language to be carried over on reply (#18557)

In most cases, replies to a toot are written in the same language as the
toot being replied to.

app/javascript/mastodon/reducers/compose.js

@@ -328,6 +328,10 @@ export default function compose(state = initialState, action) {
       map.set('preselectDate', new Date());
       map.set('idempotencyKey', uuid());
 
+      if (action.status.get('language')) {
+        map.set('language', action.status.get('language'));
+      }
+
       if (action.status.get('spoiler_text').length > 0) {
         map.set('spoiler', true);
         map.set('spoiler_text', action.status.get('spoiler_text'));