Displaying in Translations Manager the ini files from plugins' language folder

The plugin language files when only available in the plugin folder are not proposed to translation.
This PR will display them.
To test, install https://www.dropbox.com/s/c9o8sxtud7swebj/jostag_plugin.zip?dl=0
Then choose in Translations Manager an installed language (not fr-FR or en-GB) 
test before and after patch

This does not totally solve the issue as we can translate the .ini but not the .sys.ini.
It seems that one is not loaded.