In trying to make a french translation of Vanilla/Garden (still in work, but I will share it when it's done), and after having found help on this forum, I've made my own solution that is below :
1. Download and install Undefined Translation Collector
2. Enable it in the Plugins section of the Dashboard
3. Browse a maximum of pages in the conversations, discussions, and Dashboard
4. Go to the folder "plugins/TranslationCollector" and copy/paste the ApplicationName.php files into /applications/ApplicationName/locale/fr-FR/ and rename each file definitions.php
Don't forget to change "/fr-FR/" to your language like "/lang-COUNTRY/"
5. Translate each file
6. Add this lines in conf/config.php :
$Configuration['Garden']['Locale'] = 'fr-FR';
7. Delete library_mappings.php in /cache/ directory
8. Reload your community / forum and it's working fine !!!!
Sorry for this long explanation, but I think that is detailed enough ;-)
I hope this would help someone ;-)