Selma 1

Also Portuguese:

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https://www.behindthename.com/name/selma-1/top/brazil
https://forebears.io/forenames/selma

Also Finnish: https://www.nordicnames.de/wiki/Selma

As of January of this year, there are 106 bearers of this name in Poland.

Selma is a very rare name in Poland (I personally have never come across any Selmas over here), but the name has been in use nevertheless. In January of this year, 173 women in the Polish population had this name as their first name. 72 had it as a middle name. This name wasn't ranked among baby names used for girls last year.
I don't know if it's particularly more common in any specific age group here but I think most Polish people who are familiar with this name would consider it rather elderly-sounding, like a lot of rare, Germanic names.
Possible nicknames may include things like Selmi, Selka, Selmunia etc.
As far as I am aware there is no name day dedicated to Selma in Polish calendars, but since Selma comes from Anselm, a Selma could celebrate her name day in Poland April 21, together with Anzelm.

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

Also Estonian: https://et.wikipedia.org/wiki/Selma_(eesnimi)

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, 83 is the most common age for an American (U.S.) Selma who is registered female with the Social Security Administration. It is the 2, 006th most common female first name for living U.S. citizens.

The name Selma was given to 88 girls born in the US in 2015.

The name is also rather common in Iceland. [noted -ed]

Also a feminine name in Slovenia.