The source code of this example is here.
Below are two iframes. The left contains a page in this application, the right contains the settings editor provided by tzunghaor/settings-bundle and customised in this app. This way you can see your settings and their effect side by side. After you change the settings, you have to reload the left iframe (or the whole page) for your changes to take effect.
When you turn on the translation (see row below dashed line), my pseudo translator will put every translatable string in brackets and attach a flag to them.