So I found MySQL COALESCE()

$products = Product_Lang::select(\DB::raw('products_lang.*'))
                ->join('products', 'products_lang.product_id', '=', 'products.id')
                ->join('category_product', 'products.id', '=', 'category_product.product_id')
                ->where('language_code', App::getLocale())
                ->where('display', 1)
                ->where('category_id', $category->id)
                ->selectRaw('CONCAT(name, " ", model) AS product_name, products.id')
                ->selectRaw('COALESCE(discount_price, price) AS product_price')
                ->orderBy($sortby, $order)
                ->paginate(50);