We want to translate Tempo to Greek. Where can I find the necessary guidelines t

Internationalization for Forms and Dashboards plugin should help:
forum.appian.com/.../summary

Thanks for that. It's, indeed, quite helpful, but what/where is the set of the Tempo bundles I have to translate, before I am able to use the plugin in order to lookup for the translated version ?

The bundles used by that plugin are defined by the Appian designer. It comes with an application that needs to be imported, that will create a Translations folder where the bundles can be uploaded to. This plugin is primarily used to support multiple languages in application you're building.

If you would like the standard Appian Tempo user interface to be in Greek, then you can raise a support ticket requesting it be added to the product.