Add language dropdown to compose in web UI (#18420)

app/javascript/mastodon/actions/languages.js

@@ -0,0 +1,12 @@
+import { saveSettings } from './settings';
+
+export const LANGUAGE_USE = 'LANGUAGE_USE';
+
+export const useLanguage = language => dispatch => {
+  dispatch({
+    type: LANGUAGE_USE,
+    language,
+  });
+
+  dispatch(saveSettings());
+};