Scusa amico, I was so occupied with wttr format codes & fastfetch module construct I forgot to wrap curl -s 'blabla' inside echo $(), so curl gets the response from the server & says … nothing 
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Just wrap the curl command & add the entire block at the end of your fastfetch config (don’t forget to put a comma after the last entry, ie: "break",), should look like this, I just installed fastfetch & tested fine:
{
"type": "command",
"key": "Weather",
"text": "echo $(curl -s 'https://wttr.in/San%20Salvo?lang=it&format=%l,+%t+%c+%C,+%p')"
}
Output:
Weather: San Salvo, +22°C 🌦 Pioggia a tratti nelle vicinanze, 0.2mm
Apologies to all for unintended mini thread hijack, thumbs up this post if anyone wants this posted as a separate thread for reference, @Sermor if you want a longer discussion post a new thread & link to it here.