Can I choose that encoding instead of the default? I can open a file with similar apps. If I open the same file with Scratch, it displays non-readable characters. I cannot find the appropriate setting for changing that. Thank you.
First of all, check that locale it's installed. From terminal, execute
cat /usr/share/i18n/SUPPORTED | grep 8859-7
You can see an output like this:
If you want to configure your system with the first locale (el_GR ISO-8859-7) you must reconfigure your environment. Try from terminal:
localectl set-locale "LANG=el_GR.ISO-8859-7"
localectl set-locale "LANGUAGE=el_GR.ISO-8859-7"
localectl set-locale "LC_ALL=el_GR.ISO-8859-7"
Now, you must generate your desired locale:
sudo dpkg-reconfigure locales