I'd say "λεω" would translate as "speak". In any case, it's just not used. As with most languages, there are fixed ways of saying things and these are even more evident in frequently used expressions. I would not use "λεω" in this expression it would be looked upon as incorrect.
Why not 'to tell'? It is among the suggestions and it is grammatical though with a different shade of meaning.
I add, answering a comment below, that it is utterly false that 'buongiorno' in Italian is used only the morning. It is used from dawn up to 5 p.m. After that, 'buona sera' is the standard.
Here is the response for" Say or tell ?
from: English Grammar Today - Cambridge Dictionary
https://dictionary.cambridge.org › grammar › british-grammar › say-or-tell
"Say focuses on the words someone said and tell focuses more on the content or message of what someone said: 'Hello,' she said. Not: 'Hello,' she told."
Here is an important note: The Duolingo Hints function in the manner of a dictionary. Many words/phrases may be shown as definitions but each is not correct for every situation. Indeed, Duolingo goes one step further in assisting the learner by putting the preferred translation at the top. So while the hint here shows both "to say" and "to tell", "to say" is at the top.
In addition, Duolingo often gives you the full phrase or as in this case the infinitive "to say" required for the sentence.
"Καλημέρα" in Greece is used, until about noon, then we say "Καλησπέρα".
It took me a few seconds to realize what you were trying to say. But I think I have it now.
You seem to have recognized the second part of καλημέρα as meaning day and are offended that it is not "good day". Sorry, but in Greek we use it only for the morning.
But Greek is not alone see these:
Bonjour French contains "day" but is used in the morning
Buongiorno the same for Italian
Buenos días and Spanish
So, you sure do have to translate it as "good morning".
"Hello" in Greek is "γεια, γεια σου, γεια σας, χαιρετε" and can be used throughout the day. You cannot use it as a translation for "καλημέρα".
All of these can be found in the Tip & notes for the Phrases Skill or in any good dictionary ... you are free to search.
Thanks for your response. Apparently I've been misled by multiple sources, over the years. Glosbe includes "hello" among its translation of καλημέρα. Μικρό ελληνικό λεξικό states, "επιφώνημα χαιρετισμού κατά τη διάρκεια της μέρας." (As for French, I can verify that "bonjour" is often used throughout the day.)
Oh, thanks for the French hint my French is still shaky and I have zero experience with actual French people. But I can vouch for the Greek. However. of the 7 dictionaries in this link only 2 mention hello. But we may have to reconsider and add "hello" as an alternative. Personally, I would never use it after 12ish. http://www.lexilogos.com/english/greek_dictionary.htm#