I must say there is nothing special in the mod. It is just adding the languages which have just a few strings translated. The pt-PT version has nothing in it yet, but that should have no effect in my problem.
If you have selected the language pt-BR and a mod has a locale file pt.json, it should load that, unless it has a more specific locale file named pt-BR, which it should then load this one instead. If it has neither, then and only then it should load the en.json