Some progress today.
Interface language
If you want to use The Social Music Network in your language, the first step is to select it in Profile > Preferences > Interface > Interface language.
This is possible because today I have enabled these settings:
Multilingual plugin
I’m not sure what has happened, but I have found problems and inconsistencies with this plugin, and I have uninstalled it. Maybe it isn’t needed.
LibreTranslate machine translation
Now posts have a 
icon at the bottom, and if you click it, you should get an automatic translation to the language you have defined in your preferences.
I wasn’t able to make LibreTranslate and Discourse talk to each other locally in the virtual server. I decided to purchase an API key as a short term solution, while we figure out how good is the multilingual system with automatic translations. Also, how much needed it is, althouth we need to have it in place to attract speakers of non-English languages – otherwise we are building a self-fulfilling prophecy.
Please share your experiences with these automatic translations!