Make an account on https://gitlab.com/
Translate these three files in a text editor:
Note: Don't translate words inside '%' symbols, e.g. '%user%' and don't translate the descriptors e.g. 'pagination:' - See an example translation if this is unclear.
Save the files as messages.xx.yml and validators.xx.yml where 'xx' is the ISO 639-1 language code.
Go here and click 'Fork':
Once you're in your fork, click the + button and select 'new branch'. Name your new branch something like 'greektranslation'. (no spaces)
Now navigate to the following path in your fork:
Click the + button and choose 'Upload file'. Upload all three of the .yml files you made.
Go back to the main page of your fork. Click 'Create merge request'. Make sure the 'source branch' is the 'xx translation' branch you created. Click 'Submit merge request'.
There is also a translation helper tool available for people who want to update incomplete translations. (tool is no longer working.)
As new features are added to the site, the new text needs to be translated, so use the tool to check whether older translations need updating.