feat(footer): graduating language selector to production ready (#4069)

### Related Ticket(s)

#3665, #4049 

### Description

Removes footer language-only dropdown from behind feature flag.

### Changelog

**New**

- footer stories with `languageOnly` variant.

**Changed**

- update README

**Removed**

- feature flag for language dropdown

<!-- React and Web Component deploy previews are enabled by default. -->
<!-- To enable additional available deploy previews, apply the following -->
<!-- labels for the corresponding package: -->
<!-- *** "package: vanilla": Vanilla -->
<!-- *** "package: services": Services -->
<!-- *** "package: utilities": Utilities -->
<!-- *** "package: styles": Carbon Expressive, React (Expressive) -->
<!-- *** "RTL": React (RTL) -->
<!-- *** "feature flag": React (experimental) -->

packages/react/.storybook/_container.scss

@@ -46,6 +46,11 @@ body {
   height: 100%;
 }
 
+#root,
+div[role='main'] {
+  height: 100%;
+}
+
 // Storybook styles
 .storybook-center-container {
   padding: 1rem;