For my translations German to English I use this extension:
That is not a perfect one, but it eases work a good part. Simply copy the original language files, i. e. en-GB.com_bwpostman.ini, to Your language folder (i.e. fr-FR). For the correct locations and file names see here:
Then call Localize in the back end at components → localize
First time it will last a good while until it has read all the language files Localize finds in Your installation. Then you may switch to translations, filter for bwpostman and You can edit the language files.
The sys.ini files have some <i class="icon-…"> inside, that creates an error in Localize. If You comment that lines out (that is a semicolon ;
at beginning of line), You are also able to edit these files.