Detect if a language is translated or not For entries where the content isn't translated yet I'd like to add a "Sorry not translated yet" message.

Since all the content is synced is there a way to check if the synced content is the same as in the other locales?
You could try adding something like the following to the end of your template. This is assuming you only have 2 locales.

{# test for the current entry locale (i.e. spanish) #}
{% if entry.locale = 'es' %}

    {# retrieve the same entry in the original locale #}
    {% set localizedEntry = craft.entries.id(entry.id).locale('en') %}

    {# compare the body content, and append additional content #}
    {% if localizedEntry[0].body == entry.body %}
        Sorry, not translated yet.
    {% endif %}

{% endif %}