Hello,
That is related to Keyboard Layout. Check in Settings > Input devices > Keyboard and switch to Layouts tab. You can set different things there, switch the order, assign a shortcut … Test also the variants …
Depending if you installed libreoffice-fresh or libreoffice-still … you need to install libreoffice-fresh-el or libreoffice-still-el for Greek language. If you need also thesaurus and spell check, there are other steps you need to do. You need to install libmythes and mythes-el for thesaurus and hunspell-el for dictionary…
Also see LibreOffice - ArchWiki
Go step by step Locale - ArchWiki
and also check this
KDE - ArchWiki