You can certainly edit it for your own use.
If you would like to release it here at phpbb.com, you will need to become the maintainer for the translation. Go to the
Language Downloads page, click New Translations and you will see an
apply here link.
If there is already a maintainer, you will need to contact the maintainer to see if they are continuing maintaining the translation or if they are willing to turn it over to you. If there is no response from the maintainer, then read
List of current translators / how to become one.