with this

<?php

namespace App\Http\Controllers;

use App\Pays;
use Illuminate\Http\Request;
use App\Societe;
use App\Http\Requests;
use App\Http\Controllers\Controller;

class SocieteController extends Controller
{
    /**
     * Display a listing of the resource.
     *
     * @return Response
     */
    public function index()
    {
        //
        $data['societes'] = Societe::with('payss')->get();
        //$societes=Societe::all();
        $data['page_title'] = 'Sociétés';
        return view('societes.index',compact('societes'))->with($data);
    }

FatalErrorException in Model.php line 732: Class 'Pays' not found

you do realise you seem to have a type in your Pays Model?

,,, public function Payss() ,,,

notice payss