Celia

As a few other people have already mentioned, Celia is also used in Poland, although it's quite a rare name and has never been popular, Cecylia, Celina or even Celestyna are more popular. Unlike in English, in Polish Celia seems to be used exclusively as a stand-alone name rather than a diminutive of Cecylia or any other similar-sounding name, at least I don't know of such usage. Last year (2019) 2 girls were given the name Celia as a first name, and the name was #636 in the girls' first names' ranking. It was not used as a middle.
Nicknames may include: Cela, Celka, Cesia, Celunia, Celusia, Celinka, Celisia, Celcia, Celi, Celijka...
I don't know if Celia has an actual name day of its own in Polish calendars, can't think of any, but I guess a Celia could celebrate her name day together with Celestyna (6 April), Cecylia (22 November) or Celina (15 December).

Also Polish: https://pl.wikipedia.org/wiki/Celia
https://pl.wiktionary.org/wiki/Celia

Also Aragonese: https://an.wikipedia.org/wiki/Celia_Marc%C3%A9n

Also Galician: http://www.ige.eu/igebdt/esq.jsp?idioma=gl&ruta=onomast/nomes.jsp
https://gl.wikipedia.org/wiki/Lista_de_nomes_femininos_en_galego#I

In 2018, 58 is the most common age for an American (U.S.) Celia who is registered female with the Social Security Administration. It is the 1, 048th most common female first name for living U.S. citizens.

The name Celia was given to 339 girls born in the US in 2016.
More than 99.9 percent of people with the first name Celia are female.

My name is Celia and people always have trouble pronouncing it. I always get called CeCilia or Cee-lee or some variation of the two. It gets frustrating, but I love my name. It's unique, because you don't meet many Celias.

This is a Polish variant, too.