In the past we’ve used https://www.transifex.com/ . We’re just having a look around at the moment to see what else is out there …
Thanks!
We have used self hosted free software Weblate in one of our projects. It has a web interface and can integrate with the source code of an application. Don’t have anything bad to say about it once you have it setup. Non-techy translators have had no problems getting their work done on it.