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What will be the default language when the user first arrives on the home page ?

This will be the default language you mentionned in your wp-config file.

The flag icons I need are not available in the `fw-quick-langswitch/fwqls-flags` directory. Where can I find other flags ?

You can find other flags on http://www.famfamfam.com/lab/icons/flags/. Don't forget to rename them after full WordPress locales (ie : fr_FR.gif)

Does this plugin allow content translation ?

No, this plugin only uses the .mo files loaded in your wp-content/languages and eventually your buddypress and theme directories.