There are several ways to type diacritics. One, is to install the appropriate language package that comes with the system you are using (Windows, IOS, Android, etc) and then switch between the various language systems/keyboards as needed. Another, much easier is to use a browser/application that provides multiple languages support; for example, in Google Chrome, you could use Google Translate application which will provide a 'soft' keyboard for the language selected! This is by far the easiest as it should work on any system (Windows, Apple, Android) as long as you have Google Translate access in your browser... NOTE: I use Chrome which comes with Google Translate... HTH, Daniel.
Ubuntu comes with Greek keyboard where you type
; (a dead key) and then the letter - this gives you an accented letter. Actually much easier than finding extensions.
There's a bunch of other Greek layouts, too (eg. without dead keys if this is not your desired behavior).
Thank you! I should have explained better: I use the Greek keyboard that comes with Windows but I haven't found out how to insert accents. In any case I had not thought of the Google Translate solution. It sounds quite useful. Thanks again. Regards, Stefania
For use of the Greek language package in Windows, see this link: http://patristica.net/graeca/how-to-type-in-greek/. It works with Windows 10 as well. HTH, Daniel.