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Mario, hello from Athens The keyword planner tool has some peculiar functionality when it comes to Greek as you describe. Do not get fooled though - even though the tool has some issues, the google search engine itself has no problem knowing that toned and non toned greek characters are the same that that an σ and an ς are the same character (used in different cases). So to answer your question - go with properly spelled and toned words and if you prefer lowercase then that is fine, you will not be losing out on potential traffic because of which 's' is used on the end of the words, but in the kw planner tool you will want to try all potential combinations of tones and sigmas to make sure you have the best idea of traffic patterns. Hope it helps!

Τhere is a way to understand if it is ok and in Greek language? On this link -> Can my URLs use non-English words? - Google Webmaster Youtube i think that refers is general. How can i cross that its ok and with Greek language? I'm more close to put url names on Greek-lish (= English characters) with Greek meaning, and not in my language. Thank you very much