AMP for WordPress - Language switcher flags are not displayed with the standard template mode

Overview of the issue

If you are using the AMP for WordPress plugin, you will see that the flags from the WPML language switchers are not displayed on the front-end when you select the Standard template mode in AMP → Settings.

Workaround

As a workaround, please go to WPML → Languages → Language switcher options and add the following CSS in the Additional CSS text area:

.wpml-ls-legacy-list-horizontal .wpml-ls-flag,
.wpml-ls-statics-post_translations .wpml-ls-flag,
.wpml-ls-legacy-list-vertical .wpml-ls-flag {
    display: inline-block !important;
}