Thank you for the patch @thijsoo !

In addition to the suggested variable naming from @costdev, there's a few things we need to keep track of, the current proposed solution would set the value of $is_writable_template_directory to the status of the very last directory being checked, so instead it may be an idea to instead show each available directory, and if they are writable or not, as the value (this can be passed as an array, some pseudo-code for you below to demonstrate)

$is_writable_value = [];
$is_writable_debug = [];
foreach ( $theme_directories as $theme_directory ) {
    $is_writable_value[] = sprintf(
        '%s: %s',
        $theme_directory,
        ( wp_is_writable( $theme_directory ) ? __( 'Writable' ) : __( 'Not writable' ) )
    );

    $is_writable_debug[] = sprintf(
        '%s: %s',
        $theme_directory,
        ( wp_is_writable( $theme_directory ) ? 'writable' : 'not writable' )
    );
}

(Note that it needs two variables, as the debug value is never translated, for consistency between languages in debug data.)

The label for "The themes directory" should also be updated to use the _n() translation function, so that it can account for if there is only one, or multiple theme directories available.