I found a solution:
In / catalog / controller / product / product.php
somewhere after line 129 add:
$categories = $this->model_catalog_product->getCategories($product_id); if ($categories) $categories_info = $this->model_catalog_category->getCategory($categories[0]['category_id']); $this->data['category_title'] = $categories_info['name'];
This will lead to the title of the first product category (since the product can be in more than one category).
After that, in the product.tpl file echo $category_title;
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