[fenix] For https://github.com/mozilla-mobile/fenix/issues/15796 - show device language under follow device language (https://github.com/mozilla-mobile/fenix/pull/15966)

app/src/main/java/org/mozilla/fenix/settings/advanced/LocaleViewHolders.kt

@@ -39,7 +39,7 @@ class SystemLocaleViewHolder(
 
     override fun bind(locale: Locale) {
         locale_title_text.text = itemView.context.getString(R.string.default_locale_text)
-        locale_subtitle_text.visibility = View.GONE
+        locale_subtitle_text.text = locale.displayName.capitalize(Locale.getDefault())
         locale_selected_icon.isVisible = isCurrentLocaleSelected(locale, isDefault = true)
         itemView.setOnClickListener {
             interactor.onDefaultLocaleSelected()

app/src/test/java/org/mozilla/fenix/settings/advanced/LocaleViewHoldersTest.kt

@@ -71,7 +71,7 @@ class LocaleViewHoldersTest {
         systemLocaleViewHolder.bind(selectedLocale)
 
         assertEquals("Follow device language", view.locale_title_text.text)
-        assertFalse(view.locale_subtitle_text.isVisible)
+        assertEquals("English (United States)", view.locale_subtitle_text.text)
         assertTrue(view.locale_selected_icon.isVisible)
     }