Sorry for the slightly late reply.
This is my first time using it, so I'm not really an expert...
I checked what I did on my project, and if I haven't forgotten anything, you need to:
have the translated files in a “locales” folder (data\locales)
In the plugin’s settings menu, I filled in:
Source language: fr
Default language: en
Available languages: fr, en
In the translation tool, I’m not logged in—I’m doing the translation manually—but I have “French” as the source and “English” as the target.
I don’t know if this helps....