temp_user1999
In the case of Gboard, by default it only comes with English spellcheck. In order to use foreign languages, you need to have network access enabled, switch to the particular language you would like to use, and then Gboard will download the dictionary of that language in the background. It should be pretty quick on a normal internet connection. If you afterwards disable the network connection, it will still work. Of course, if there are updates to the spell checker, it will not get updated without internet access (I'm not sure how often, if ever, dictionaries are updated by Gboard).
other8026
In the case of the GrapheneOS stock keyboard, I don't think Greek is a supported language for spell check, regardless of the latest GrapheneOS release (2023103100, which is what I have installed). If you go to Settings \ System \ Keyboard \ Spell Checker, and select "Languages", Greek doesn't show up as a language when you have the system "Spell Checker" selected as the Default spell checker. On this latest Android v14 release of 2023103100, only the following languages appear on the list in my phone:
English US, English UK, French, German, Italian, Spanish Russian, Portuguese, Dutch, Czech
@other8026 does your phone show any other languages after the latest update?