How to translate this array of this class

I am creating a theme for wordpress and I would like it to be multilanguage. I am using this wordpress function: link that allows you to translate texts.

The problem comes in the use of this class:

class haceTanto extends DateTime {

    protected $strings = array(
        'y' => array(__('1 year', 'mytheme'), __('%d years', 'mytheme') ),
        'm' => array(__('1 month', 'mytheme'), __('%d months', 'mytheme') ),
        'd' => array(__('1 day', 'mytheme'), __('%d days', 'mytheme') ),
        'h' => array(__('1 hour', 'mytheme'), __('%d hours', 'mytheme') ),
        'i' => array(__('1 minute', 'mytheme'), __('%d minutes', 'mytheme') ),
        's' => array(__('1 second', 'mytheme'), '%d seconds'),
    );

    public $profundidad;

    public function __construct( $fecha,$profundidad='i')
    {
        parent::__construct( $fecha );
        $this->profundidad = $profundidad;

    }


    public function __toString() {
         try 
        {  
            $now = new DateTime('now');
            $diff = $this->diff($now);

             $text = '';

            foreach($this->strings as $key => $value){

                $dato=$this->getDiffText($key, $diff);

                if($dato!=null ){ $text .= ' '.$dato; break; }

                if( ($text .= ' '.$this->getDiffText($key, $diff)) ){

                }
                if($this->profundidad == $key) break;
            }
            return $text;   
        } 
        catch(Exception $e) 
        {  
            trigger_error($e->getMessage(), E_USER_ERROR);  
            return '';  
        }  


    }

     protected function getDiffText($intervalKey, $diff){
        $pluralKey = 1;
        $value = $diff->$intervalKey;
        if($value > 0){
            if($value < 2){
                $pluralKey = 0;
            }
            return sprintf($this->strings[$intervalKey][$pluralKey], $value);
        }
        return null;
    }
}

As you can see, I use __ in the array

'y' => array(__('1 year', 'mytheme'), __('%d years', 'mytheme') ),

but gives this error:

Fatal error: Constant expression contains invalid operations in C: \ laragon \ www \ mul \ wp-content \ themes \ mytheme \ inc \ class.time.php on line 5

This I guess it is, because the class below uses sprintf

return sprintf($this->strings[$intervalKey][$pluralKey], $value);

And that's why the problem but I do not know how to solve it in order to translate all the chains of year, month, day ... in the plural and in the singular.

I have tried to explain myself as well as possible, I hope someone can help me.