Translating a site where default is in two languages Drupal is written in English. We have created some custom modules where the strings in the module are written in French. These are passed through the t() so can be translated.

I am a little confused how I can show my site in both French and English when the default site language English is in both language. Please could you point me in the correct direction?
When you pass string through t() without specifying a langcode, then it will default back to English.
So, if string is a French word, it will be handled as an English word.

Two options:


Always use English words in t() and use the translate interface to
translate it to French. 
As per https://api.drupal.org/api/drupal/includes!bootstrap.inc/function/t/7
set the langcode as such t($string, $args = array(), $options =
array('langcode' => 'fr'))