Sorry, i made a mistake, i used var_dump(Lang::get('hierarchy')) which returned an empty array and i did var_dump(Lang::get('hierarchy.value') which returned null.
Unit tests run at the command line, so there is no URI, and no language to get from it. The method described in the blog post only works for interactive requests.
You will have to make sure the default language is set correctly in your app/config/config.php (either to 'en' or 'fr').
I load the language file in my library's constructor.
The var_dump() is done in my library's function.
in app/lang/fr/... there is nothing because I would like to use my lang file in a module which look likes modules/hierarchy/lang/fr/... and there is also a file in modules/hierarchy/lang/en/...