The translated PO/MO files should be uploaded in /wp-content/languages/plugins/ so your language files will never been overwritten. WordPress uses files from that directory.

I have a couple of issues thiough in using the PO files.

In the wpforo-it_IT.po some language strings are missing (I mean the original english language strings). I can find those strings int he "phrases" section in backend, but not in the .po file.

When I update wpforo, the .po and .mo files get overridden and I have to upload again the edited ones, is there a way to solve this?

Thank you

You can't use the phrase system for multi-language websites. If your website is a multi-language you should only use the PO/MO system. The wpForo Phrase system is designed for quick changes of phrases or for quick translation of front-end phrases to one language you're currently using on your website. 

Now That I see it, there is only "english" listed there as a language, how do I add another language? (see https://www.screencast.com/t/yBe1PLOa)

Thanks

All wpForo phrases are located in Dashboard > Forums > Phrases admin page. You can find and change each phrase as you like.

If you don't find the phrase, please, click the button, insert the identical English version of that phrase as Phrase Key and the translated phrase as Phrase Value.

I am trying to translate the system messages that appears in a popup in the top right side of the screen (like "invalid email", "working..." ) etc.
But I can't find those language strings in the language file, are they supposed to be there?

Thank you