what should be the quality of a translation agency?

The quality of the translation work an agency delivers will vary based on their interpretation of what amounts to quality work

A quality agency will have a pool of talented freelancers across the globe with industry specific experience that they can call upon for specific projects. A good agency for example AdGloriam adgloriam-translations.com/ , will have a robust qualification and vetting process for translators, this should be demonstrable to potential clients.