Use translated taxonomy labels in plugin

You have to use a plural aware function like _n() or _nx(), because the words around the number might change in some languages depending on the amount items.
For visible numbers use number_format_i18n() and %s, not %d.

You cannot reuse the labels, because plural forms change depending on context in some languages.

This is how I would write your example:

// bare number, needed for _n()
$num  = count( $created );

// Singular or plural, we use %s, because we don't know what 
// number_format_i18n() will return.
$text = _n(
    'Created %s category',
    'Created %s categories',
    $num,
    'unique_plugin_textdomain'
);
// thousands separator etc.
$display_number = number_format_i18n( $num );
// Finally, the result:
$message        = sprintf( $text, $display_number );