Nell

While the standard form of this name in Polish would be Nela or Nella, Nell has also had some usage in Poland. That's probably because in the book In Desert And Wilderness by Henryk Sienkiewicz, the protagonist's name is Nel Rawlison (she is English, I don't really understand why it's spelt Nel rather than Nell but that's just how it is), and this is one of the few acceptable Polish feminine names not ending in -a which we were allowed to use when we had stricter naming rules. Also, names like Nel, Nela, Nella, Nelia, have been quite trendy lately, maybe now not so much but some ten years ago I'd say it was definitely a trend. Also Kornelia is a more formal name which has been gaining a lot of popularity lately, as well as vintage Aniela (which is also often nicknamed to Nela). That all may be the reasons why Nell is also getting some use.
Last year (2020) 14 girls were given the name Nell as a first name. For a comparison, 191 girls were given the name Nel, and 1731 were named Nela. In the whole population, in January of this year, there were 480 Nells in Poland with this name as a first name, and 88 women had it as a middle name.
A Nell in Polish could of course be nicknamed to Nelly or Nellie, or Nelli/Neli, but also Nela, Nelka, Nelusia/Lusia, Nelunia/Lunia, etc. Etc. She could celebrate her name day together with Aniela (January 4 and 27, March 28, May 30 and 31, November 18), Kornelia (March 31, September 16), Petronela (May 31), Helena (March 2, May 22, July 31, August 18), Lena (November 10) etc.

In 2018, 78 is the most common age for an American (U.S.) Nell* who is registered female with the Social Security Administration. It is the 1, 977th most common female first name for living U.S. citizens.

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*as a first name, not a nickname.

The name Nell was given to 61 girls born in the US in 2015.