Ilona

Also used in Dutch: https://www.meertens.knaw.nl/nvb/naam/is/Ilona

Also French (Belgian) and Flemish: http://www.names.be/meisjesnamen.html?met=Ilona&sort=alpha

Also used in French: https://meilleursprenoms.com/popularite-prenom/Ilona

Also Dutch: https://www.meertens.knaw.nl/nvb/naam/is/Ilona

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Also French: https://meilleursprenoms.com/popularite-prenom/Ilona

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Also Russian, Belarusian, Serbian, Romanian and Slovak: https://forebears.io/forenames/ilona

Also Banatswabian: https://ofb.genealogy.net/famreport.php?ofb=zichydorf&ID=I17190&nachname=Czikora&modus=&lang=de
https://ofb.genealogy.net/famreport.php?ofb=zichydorf&ID=I15182&nachname=Dudas&modus=&lang=de

Also Croatian: https://actacroatica.com/en/name/Ilona/

Also Slovene: https://sl.wikipedia.org/wiki/Ilona

Also Moldovan: https://ro.wikipedia.org/wiki/Ilona_Sp%C4%83taru

Also Estonian: https://et.wikipedia.org/wiki/Ilona [noted -ed]

Also used in Ukrainian, spelled Ілона : http://www.euam-ukraine.eu/news/multimedia/meet-Ilona-ukrainian-public-servant-of-the-new-generation-in-euam-partner-story/ -- "Ilona Havronska from the Ministry of Justice of Ukraine".
You'll find numerous bearers on social media.

With 168 415 bearers, Ilona is the 5th most common feminine name in Hungary. (01/01/2016).

Ilona is also German, Polish, Serbian, Croatian, Czech and Slovak form. [noted -ed]

It's a Lithuanian name also.

Also used in Lithuania. [noted -ed]