bbPress.org

The message in the .po is

‘This user…’

but in your example it is

‘you are…’

The code should be translated

but you can just put this in your functions file

//This function changes the text wherever it is quoted
function change_translate_text( $translated_text ) {
	if ( $translated_text == 'You are not currently subscribed to any forums' ) {
	$translated_text = 'new text';
	}
	return $translated_text;
}
add_filter( 'gettext', 'change_translate_text', 20 );

and change ‘new text’ to whatever it should say

@fabioweb

Ciao Fabio,
As you can see on https://translate.wordpress.org/locale/it/default/wp-plugins/bbpress , bbPress is translated in stable and dev at 100%, so I confirm that all strings have been translated in Italian.

What you show in your screenshots is however coming from a theme or an extra plugin and not from bbPress.

So identify the theme or plugin that adds this string and translate it. More information on how to translate can be found on https://it.wordpress.org/traduzioni/