I typically have my keyboard set up to use multiple alphabets, toggling with alt+shift; I use American English and Greek most frequently, followed by occasionally Japanese with Anthy, and occasionally Cyrillic. It works very well in almost every program.
However, Scribus, which is otherwise very decent layout software, refuses to recognize Greek. I really need that λ! Am I missing a package, or is there something else I need to do? The space isn't even filled with a filler character, it just remains blank.
This alphabet is awfully important to engineers, so I would be very surprised if there was no solution.
Update: I have determine that the π key consistently works fine, apparently on all fonts; but even on Neohellenic, it is the only one. Is something weird going on with font processing?
I have now attempted this on two completely physically separate machines, with different hardware, and gotten the same result. I typically use alt+shift to switch layouts, via Mint 19's keyboard manager. Doing so works on literally every other program I've tried, on both machines. However, Scribus, on both machines, ceases accepting input until I switch back to en mode. In gr, the only key that works is π; all others have no effect.
Symbol insertion works reasonably well, but this is a non-solution if I'm to attempt to type anything in the Hellenic alphabet.
I'm pretty sure that Scribus does work with the Greek keyboard layout. (But I cannot test, since I don't have the keyboard layout installed and cannot write Greek...)
Luckily, in a side note, you describe your issue as: "it stays blank". In that case, I guess that the default font (or the font you have picked does not contain the Greek glyphs (letters). Can you try with a different font? If possible, a boring one with lot of glyphs...
And if you only need a lambda (or single "scientific" letters from the Greek alphabet) you can always use "insert > glyph"... (you can create a set of often used letters in there...)
In your coments below you have now specified the font you are using and I got tempted to try it out. (The hardest part being: find out how to switch keyboard when you have no Desktop Environment...)
I have a Debian Testing system with the current stable release of Scribus and this is the result:
This should be pretty close to what should be able to achieve with Ubuntu.
My conclusion: Scribus does work with a Greek font...
With the information you have currently provided, I don't see a reason why it should not work on your system, too.
So, you need to provide further information, or try to get it to work with the mindset that on the Scribus side everything is fine (sometimes, when one has doubts about the outcome, some small "errors" sneak in...).
Just one more information from my side: I've switched the keyboard layout with
setxkbmap -layout gr: You might want to try this, in the case your keyboard layout manager does not work well with Scribus...