MySQL Grouping Order
You can use ORDER BY inside GROUP_CONCAT() function. You can also change the separator, if you don't want to use comma. Further details in MySQL documentation: GROUP_CONCAT() 

Example:

SELECT 
    product_id,
    GROUP_CONCAT(name 
                 ORDER BY language_id
                 SEPARATOR ','
                ) AS product_names
FROM
    products_languages
GROUP BY
    product_id ;